docs: record Gamma-W1-T7 in PLAN.md, the track that landed without an entry

docs/PLAN.md

@@ -1390,6 +1390,58 @@ changes duration, so a resampled read is an explicit duration control — but th
 
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+#### Γ-W1-T7 — `psola-preserve`
+
+**Goal.** Preserve's splices become pitch-synchronous: the source's fundamental period is
+detected offline at sample load, cached, and the splice jump becomes a whole number of that
+period, so an aligned landing point exists by construction rather than being searched for.
+
+**Origin — Daniel's ruling, 2026-08-01, after Γ-W1-T5 landed.** T5's measurement pass showed
+the shipped fixed 50 ms OLA window with a ±window/4 correlation search could only reach
+landings spanning `[¾w, 1¼w]` — a 1.5:1 span that cannot contain a whole number of periods for
+low material. A 30 Hz tone (1470 frames at 44.1 kHz) had no phase-aligned landing at all. A
+second, distinct failure mode: splices recur every `window/|rate − shift|` frames and fail
+when that interval is shorter than the output period. Three cheaper options were offered and
+declined: widening `maxLag` to `window/2` (fixes the geometry, not the cadence); sizing the
+window from the note's known fundamental at note-on (fixes both, but trusts root-note
+tagging); enlarging the window to ~200 ms (fixes both, smears transients — the OLA crossfade
+is `window/4`).
+
+**Why this is a new track and not an amendment to Γ-W1-T5.** It touches the sample-load /
+analysis path, which is outside T5's stated surface boundary (`pitch_shift`, the stretcher
+module, `voice`'s Preserve read path).
+
+**Why PSOLA is affordable here.** ReaSampler is a sampler, so the source is fixed and fully
+known at load. Detection runs once during the reload that already happens, entirely off the
+audio thread — the usual real-time objection to PSOLA does not apply.
+
+**Surface boundary — owns:** a new pure module `core/instrument/engine/period_detect`
+(two-pass YIN, with its own `period_detect_tests` target), `pitch_shift`'s jump geometry, the
+load-time hook in `map/sample_map`'s `buildSampleData`, and `voice`'s note-on. Adds no
+`ComponentState` field and takes **no rung of the payload ladder** — a detected period is
+derived from the audio, so it is cache, not state.
+
+**Behavior and constraints.**
+- **Enforcement is by link graph, not by convention.** The detector runs off the audio thread
+  because `sampler_core` does not link `period_detect` — no translation unit on the render path
+  can name `detectPeriod`. An unknown period restores the fixed-window geometry byte for byte.
+- **Dependency it discharges:** it gates the Rate control (Γ-W2-T1) on the plan's own stated
+  principle that Rate must not ship before its Preserve engine.
+
+**Status — landed, with open findings.**
+- The **geometry** failure mode is closed and asserted.
+- The **cadence** failure mode is **not** closed. It was re-characterized rather than fixed:
+  the previously-headlined 7–21 % artifact-energy readings turned out to be ~95 % the
+  measurement's own spectral leakage, leaving a real excess of 0.23–0.51 %. The track asserted
+  no-regression there rather than claiming an improvement.
+
+**Open questions.**
+- **Unresolved review findings, not a design fork.** A later review of the follow-up fold
+  returned three unresolved Major findings; remediation has not yet been dispatched. The
+  findings themselves live in the review, not here.
+
+---
+
 ### Γ-W2 — New controls, and the overlay's marks
 
 **Depends on Γ-W1 for — four dependencies, two of them new:**