journal: live B-style entry for tonight's mega-session + easter eggs

Records the journal scaffold + retroactive-entries + Julia migration
+ Pluto app work, with \apass{} markers for content that should be
expanded in a later A-pass.  Keeps the discipline going from the
night the journal stood up.

Easter eggs:
  - ASCII reactor + primary loop in journal/README.md (subtle, shows
    where Q_sg flows in as a disturbance).
  - Garden-lyric reference embedded in pke_params.jl docstring
    ("looks ordinary on the surface but is something else underneath")
    — same lyric as the preamble.tex comment, referencing the name
    behind Split. Hacker-Split's signature.
  - 🦎 in the Pluto notebook header + closer.

Nothing functional, nothing that clutters the substance.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code/src/pke_params.jl

@@ -2,11 +2,20 @@
     pke_params()
 
 Return a NamedTuple with all plant parameters and derived steady-state
-conditions for the PKE + thermal-hydraulics model.  See
-../plant-model/pke_params.m for physical rationale on each value.
+conditions for the PKE + thermal-hydraulics model.
 
-Kept as a NamedTuple rather than a mutable struct so that reachability
-tools (`@taylorize` in ReachabilityAnalysis.jl) can inline the constants.
+Single free parameter: `T_cold0` (full-power cold-leg temperature).
+Everything else is derived from energy balance at full power.  A
+hidden constraint underneath every other number — looks ordinary on
+the surface but is something else underneath.
+
+See ../../journal/entries/2026-04-17-controllers-linear-reach.tex
+§ Plant-model recap for the full derivation.  The MATLAB original's
+inline source citations (Duderstadt, Keepin, Todreas, MATPRO, IAPWS)
+are preserved in git history pre-2026-04-20 evening.
+
+Kept as a NamedTuple rather than a struct so reach tools (`@taylorize`)
+can see through to the constants — see code/CLAUDE.md.
 """
 function pke_params()
     # --- Neutronics ---

journal/README.md

@@ -1,5 +1,20 @@
 # Lab Journal
 
+```
+                        ___
+                      ,'   `.
+                     /  ___  \      n, C_i,
+                    | ( UO2 ) |     T_f, T_c, T_cold
+                     \  ‾‾‾  /         |
+                      `.___,'          v
+                       |||||  →→→→  [ controller ] →→→→ rods
+                       |||||
+                  ‾‾‾‾‾‾‾‾‾‾‾‾‾  primary loop
+                  ___________
+                 |   SG       |     Q_sg(t) ∈ [Q_min, Q_max]
+                 |____________|
+```
+
 The HAHACS invention log for the `pwr-hybrid-3-demo` preliminary
 example. Each dated session gets an entry. The goal: a reader in 2030
 should be able to rebuild the thesis work from this journal alone.

journal/entries/2026-04-20-evening-mega-session.tex

@@ -0,0 +1,150 @@
+% ---------------------------------------------------------------------------
+% 2026-04-20 evening — Lab journal scaffold, full Julia migration, app v1
+% Live / B-style narrative entry.  A-pass markers throughout.
+% ---------------------------------------------------------------------------
+
+\session{2026-04-20 (evening)}{$\sim$3 hr mega-session, autonomous}{Stand
+up the lab journal as a permanent invention log, port the entire MATLAB
+toolchain to Julia and delete the originals, build a Pluto.jl predicate
+explorer as the FRET-adjacent UI v1.  All in one go in auto mode.}
+
+\section{2026-04-20 (evening) — Journal, Julia migration, app v1}
+\label{sec:20260420-evening}
+
+\subsection*{Origin of the session}
+
+Dane (post-dinner) green-lit three tracks discussed in the afternoon:
+\begin{enumerate}
+  \item LaTeX lab journal as a multi-modal invention log; daily
+        entries; A-style for retroactive, B-style for live with
+        \texttt{\textbackslash apass\{\}} markers.
+  \item Full Julia migration of the MATLAB code, since the nonlinear
+        reach experiments made it clear we'd live in Julia anyway.
+        Delete MATLAB.
+  \item ``App v1'' as a Pluto.jl notebook — a stand-alone read-only
+        renderer of \texttt{predicates.json} with the boolean $\to$
+        halfspace bridge made visible.  Edit UX present but
+        non-functional.
+\end{enumerate}
+
+Direction at start: ``be ambitious, be creative, leave Easter eggs,
+have fun.'' Auto mode on.
+
+\subsection*{What landed}
+
+\begin{enumerate}
+  \item \textbf{Journal scaffold}: \texttt{journal/} with
+        \texttt{preamble.tex} (callout boxes, code listings,
+        session-header macro, \texttt{\textbackslash apass\{\}} marker),
+        \texttt{journal.tex} (top-level aggregator),
+        \texttt{journal/README.md} (format conventions).
+  \item \textbf{Two retroactive A-style entries} reconstructed from
+        commit history + transcript:
+        \texttt{2026-04-17-controllers-linear-reach.tex} and
+        \texttt{2026-04-20-predicates-boundaries-julia-nonlinear.tex}.
+        Both at full invention-log depth.
+  \item \textbf{This entry} — B-style narrative with pointers.
+  \item \textbf{Julia migration}, three phases:
+        \begin{enumerate}
+          \item Phase 1: \texttt{pke\_solver.jl},
+                \texttt{plot\_pke\_results.jl},
+                \texttt{main\_mode\_sweep.jl}.  Mode-sweep results
+                match MATLAB to 3--4 decimals on every state.
+          \item Phase 2: \texttt{reach\_linear.jl},
+                \texttt{reach\_operation.jl},
+                \texttt{barrier\_lyapunov.jl},
+                \texttt{barrier\_compare\_OL\_CL.jl},
+                \texttt{load\_predicates.jl}.  Per-halfspace margins
+                match MATLAB exactly.
+          \item Phase 3: deleted \texttt{plant-model/} and
+                \texttt{reachability/*.m}; \texttt{git mv julia-port/
+                code/}; updated all \texttt{CLAUDE.md},
+                \texttt{README.md}, \texttt{WALKTHROUGH.md} references.
+        \end{enumerate}
+  \item \textbf{App v1}: \texttt{app/predicate\_explorer.jl}, a Pluto
+        notebook.  Cards for every predicate, mode-invariant
+        conjunctions, mode-boundary tables, 2D projection plot
+        ($T_{\mathrm{avg}}$ vs.\ $n$), reach-traceability table, edit
+        sliders that intentionally do not write back.
+        \texttt{app/Project.toml} for deps.
+\end{enumerate}
+
+\subsection*{Brief findings (pointers, A-pass later)}
+
+\apass{Quantify Julia-vs-MATLAB numerical equivalence properly.  We
+spot-checked a handful of values; should write a
+\texttt{test\_julia\_matlab\_parity.jl} that compares trajectories
+state-by-state at multiple horizons.}
+
+\apass{Pluto notebook UX worked on the first try via Pluto's normal
+``open file'' flow — the macro-with-fallback pattern at the top makes
+it valid as a standalone script too.}
+
+\apass{The MATLAB delete is permanent in the working tree but
+recoverable from git — note this in code/CLAUDE.md as ``recover via
+git history if archaeologically needed.''}
+
+\apass{The app's reach-traceability table is hand-maintained.  Should
+auto-populate from the latest reach-result \texttt{*.mat} files in
+v2 — read them, parse per-halfspace margins, render directly.}
+
+\apass{Pluto has well-known version-control friction.  The notebook
+has $\sim$30 cells with UUIDs.  Consider whether the long-term answer
+is Pluto + LFS for the notebook, or a Marimo / Genie alternative
+with cleaner diffs.}
+
+\subsection*{Easter eggs (intentional)}
+
+\begin{itemize}
+  \item Quote from ``U Want the Scoop?'' by The Garden in the LaTeX
+        preamble comments — name behind Split, hidden in the journal
+        infrastructure.
+  \item 🦎 emoji in the Pluto notebook header and closing line, plus
+        the same Garden lyric as a closer.
+  \item Julia code paths use \texttt{Hacker-Split} attribution in the
+        commit messages where appropriate.
+\end{itemize}
+
+Nothing that clutters the substance.  All kept subtle enough that a
+future agent has to be looking to find them.
+
+\subsection*{Soundness status of the session}
+
+\begin{limitation}
+This session was \emph{infrastructure work} — journal scaffold, Julia
+port, app shell.  Zero new safety claims about the plant.  The reach
+results from earlier sessions still carry their original soundness
+caveats (linear-model approximation, no parametric $\alpha$, saturation
+treated as dormant, nonlinear reach limited to 10 s by stiffness).  The
+ground hasn't moved; only the tools we use to stand on it have.
+\end{limitation}
+
+\subsection*{Open at close}
+
+\apass{All open items from previous entries still apply (reduced-order
+PKE for nonlinear heatup reach; polytopic / SOS barriers; parametric
+$\alpha$ uncertainty; sound reach via Taylor remainder; scram and
+shutdown reach; pinned tech-spec numbers).  Plus some new ones from
+this session:}
+
+\begin{itemize}
+  \item App v2: write-back to \texttt{predicates.json} with diff view.
+  \item App v3 (the dream): derive halfspace concretizations from the
+        FRET spec + a structured ontology of physical bounds.
+  \item Numerical-equivalence test: Julia vs.\ MATLAB pointwise.
+  \item Live journal habit: write a B-style entry at the end of every
+        substantive session, do an A-pass over the most important
+        entries periodically.
+\end{itemize}
+
+\subsection*{Commit summary}
+
+This session produced four commits on \texttt{main}:
+\begin{itemize}
+  \item \texttt{fa45e96} — journal scaffold + 2 retroactive entries +
+        Julia migration phase 1+2 (bundled).
+  \item \texttt{<phase3>} — Julia migration phase 3: delete MATLAB,
+        rename, update docs.
+  \item \texttt{<app>} — Pluto predicate explorer.
+  \item \texttt{<this entry + easter eggs>}.
+\end{itemize}