Archive Milestone 1 in docs

Move completed M1 (bank_model core) from PLAN.md to COMPLETED.md; record
the parseInt int64-to-int narrowing as a future-hardening note.

COMPLETED.md

@@ -30,3 +30,26 @@ build` is green with the new `bank_model` + `peaks` suites present.
 - [x] Refresh `README.md` layout/next-step sections to the sampler module set.
   (Landed-work reflection is doc-keeper's; this point exists so the stale MPE
   README does not mislead the first implementer.)
+
+---
+
+## Milestone 1 — bank_model + JSON round-trip (pure)
+**Goal:** The `Sample` metadata struct and `BankIndex` (add / remove / query /
+tier moves / dedup-by-hash) with JSON serialize/deserialize to `std::string`.
+CONTEXT.md §Data model, §Module architecture.
+**Verify:** CTest green. Round-trip is lossless (deserialize(serialize(x)) == x)
+across all fields; dedup-by-hash and tier filtering asserted; **relative paths
+only** invariant enforced at the model boundary (no absolute path accepted/stored).
+
+- [x] Define `Sample` with the full field set (id, display name, relative path,
+  source mode, source range in project time + PPQ, track GUID(s), wet/dry,
+  channels, SR, length sec + beats, capture tempo, optional key, peak/RMS/LUFS,
+  clip flag, tier, content hash, provenance, created ts). CONTEXT.md §Data model.
+- [x] `BankIndex`: ordered collection keyed by id; add / remove / query.
+- [x] Hash lookup for dedup-by-content-hash.
+- [x] Tier model (scratch | archive) + tier-move + tier filtering; scratch marked
+  auto-prunable.
+- [x] JSON serialize/deserialize to/from `std::string`.
+- [x] Tests: full-field round-trip lossless; dedup collapses equal-hash adds;
+  tier filter/move correct; relative-path invariant rejects absolute paths;
+  empty-index and malformed-JSON edge cases.