Phase 2: domain layer with tests

Vite + React + TypeScript + Vitest. No router, state library or UI kit.

The 159-row combo table is extracted from reference/index.html by
scripts/extract-reference-data.mjs rather than retyped: it balances
brackets past string literals to slice out the pure-data declarations and
evaluates them in a vm with no DOM. raw.ts and labels.ts are generated.
The extracted count matches the 159 the Phase 1 app reported on device.

Entry.id keeps the reference's format verbatim (shift>p1@S#FIX VEL),
built from the unsplit context string, so drill progress already on the
phone migrates instead of being orphaned.

The domain is pure where the reference was not: boxes go in and new
boxes come out, and the random source is injected. That is what made
card selection testable.

Two bugs found by doing so:

- pickCard returned the card just answered when random() gave exactly 0.
  A zero-weight card does not move the accumulator, so "r <= 0" matched
  it. The reference has this; at one draw in 2^53 it is invisible in use,
  but a pinned random source hits it every time. Fixed to "r < 0".
- boxDistribution incremented through an index that
  noUncheckedIndexedAccess types as possibly undefined.

65 tests. Several assert numbers observed on the device: 159 entries,
sample-mode layers of 3 / 35 / 2, a 16-card SHIFT+pads deck.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -156,6 +156,48 @@ now empty and bar styling is configured in `capacitor.config.json`
 installs over Phase 1's without an uninstall and drill progress survives.
 `appId` is `dev.ncode.sp404layers` and must not change for the same reason.
 
+## Phase 2: the React port
+
+Stack: **Vite + React + TypeScript + Vitest**. No router, no state library, no UI
+kit — the app is four tabs and a reducer, and none of those earn their keep.
+
+| Command | What it does |
+|---|---|
+| `npm test` | Run the domain suite once |
+| `npm run test:watch` | Watch mode |
+| `npm run typecheck` | `tsc --noEmit` |
+| `npm run extract:data` | Regenerate the combo table from `reference/` |
+
+```
+src/domain/
+  types.ts     hand-written: RawEntry, Entry, ContextCode, GroupId
+  raw.ts       GENERATED — the 159-row combo table
+  labels.ts    GENERATED — control labels, groups, contexts
+  entries.ts   derives Entry[] from RAW
+  layers.ts    which combos exist for a context + held modifiers
+  drill.ts     decks, Leitner boxes, answer checking, card selection
+```
+
+**The combo table is never transcribed by hand.** `scripts/extract-reference-data.mjs`
+lifts the literals out of `reference/index.html`, evaluating only pure-data
+declarations in a `vm` with no DOM, and writes `raw.ts` and `labels.ts`. Re-run it
+after any reference change; never edit those two files. Retyping 159 rows is
+precisely where silent errors would creep in, and the extraction cross-checks:
+159 entries, matching the count the Phase 1 app reported on the device.
+
+**`Entry.id` keeps the reference's format verbatim** (`shift>p1@S#FIX VEL`) — built
+from the *unsplit* context string and the target as authored. That string is the
+key drill progress is stored under, so preserving it byte-for-byte means existing
+boxes on the phone migrate to the React app instead of being orphaned.
+
+**The domain layer is pure.** The reference mutated a module-level `store` and
+called `Math.random()` inline; here boxes go in and new boxes come out, and the
+random source is injected. That is what makes card selection testable at all.
+
+Tests assert against numbers **observed on the device**, not just against the
+code: 159 entries, sample-mode layer counts of 3 / 35 / 2 for no-modifier /
+SHIFT / SHIFT+REVERSE, and a 16-card SHIFT+pads deck.
+
 ## Visual spec (settled — build Phase 2 against this)
 
 The look is **Direction A, "manual"**, after Teenage Engineering's EP-133 manual,