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  deleted (offset-indirect arm stays); the two superseded M5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/decisions/003-schema-annotations.md

@@ -136,9 +136,9 @@ reserving worst-case space.
 
 - `true` is a shorthand for the default (length-prefixed). This keeps
   the common case concise and the override explicit.
-- The `encoding` annotation and `maxLength` apply to all variable-length
-  types: `AlkType:String`, `AlkType:Bytes`, `AlkType:Array`,
-  `AlkType:Record`, `AlkType:Timestamp`.
+- The `encoding` annotation and `maxLength` apply to the variable-length
+  primitive types `AlkType:String` and `AlkType:Bytes`. (`maxLength` on
+  records was amended out by review #006 N3/M5 — see §3a.)
 
 ### 3a. TRecord value type
 
@@ -164,8 +164,13 @@ the `"values"` property in the schema:
   the value's size is determined by its kind (fixed-size kinds have a
   known size; variable-length kinds carry their own length prefix).
 - The count and key-length prefixes respect the schema's endianness.
-- In aligned static mode with `maxLength`, the entire record is reserved
-  at `maxLength` bytes (zero-padded).
+- ~~In aligned static mode with `maxLength`, the entire record is
+  reserved at `maxLength` bytes (zero-padded).~~ **Amended (review #006
+  N3/M5, 2026-09-02):** `maxLength` is rejected at parse on record
+  fields. The aligned materializer walks the record's inline
+  count-prefixed form and never honors the reservation (M5: silent
+  cross-field corruption), and no packed consumer enforced it either
+  (N3: silently unenforced). `maxLength` is `string`/`bytes`-only.
 
 ### 4. TUnion discriminators