Resolve N3: maxLength is string/bytes-only (review #006)

- Parse gate in BastField::parse: maxLength on any kind other than
  string/bytes is a clean Schema error (records, arrays, inline
  structs, refs, union shared fields all covered; the choke point
  needs no ref-following since $defs entries are struct/union/enum)
- Meta-schema FieldDef: if kind in {string, bytes} else maxLength
  forbidden — the published alk.dev/bast/v1 contract matches the
  parser (N2 dual-layer pattern)
- M5's compute-side record maxLength arm became unreachable and was
  deleted (offset-indirect arm stays); the two superseded M5
  maxLength tests rewritten as the n3_* parse-rejection family
- ADR-006 remedy message tailored per kind: for records both
  annotated remedies are dead ends, so the error text points at the
  last-position fix only
- Docs aligned: bast-format.md (FieldDef meta-schema + FieldDef/
  Variable-Length Encoding prose), layout-engine.md (Strategy 2 +
  ADR-006 paragraph), schema-layer.md, ADR-003 §2/§3a amended,
  builder .max_length() doc
- Review #006: N3 resolved (all findings now closed), M5 update
  note, test-count bookkeeping note (in-session probes vs static
  counts), status lines flipped to fully resolved

Verified: 547 tests green + 2 ignored doctests in BOTH release and
default profiles (a stale debug artifact from an earlier session
masked one H3 roundtrip test in debug; clean rebuild passes both),
clippy -D warnings clean, cargo doc --no-deps zero warnings, wasm
build green.

docs/architecture/schema-layer.md

@@ -230,7 +230,10 @@ type-level properties. The concrete BAST shapes are in
 The `maxLength` keyword is *not* a BAST invention — it is the standard
 JSON Schema `maxLength`, repurposed as a byte-length cap. In aligned
 mode it reserves a fixed-size slot; in packed mode it is a validation
-constraint only. See [bast-format.md §Variable-Length
+constraint only. It applies to `string`/`bytes` fields only: the parser
+rejects it on any other kind (review #006 N3 — elsewhere it was
+silently unenforced), and in aligned mode a record reservation was
+silently corrupt (review #006 M5). See [bast-format.md §Variable-Length
 Encoding](bast-format.md#variable-length-encoding) and
 [ADR-003](decisions/003-schema-annotations.md).