OffsetMap carries LeafMeta; read/write_field dispatch on it (ADR-012 §2b, plan phase 5)

Prerequisite (review #005 M2): Hash added to Endian/VariableEncoding
derives (additive; fieldless Eq enums), and to ByteRange.

- New public types LeafMeta { kind, encoding, endian } (Copy + Eq +
  Hash) and OffsetEntry { range, meta } (start()/end() accessors),
  re-exported from lib.rs. Deferred decision 2 resolved: struct —
  get(path) -> Option<&OffsetEntry>, iter() -> (&str, &OffsetEntry).
  Storage: Vec<(String, OffsetEntry)>.
- LeafMeta computed at compute time with effective endian threaded
  through the aligned walk (container default -> field override,
  propagated into nested-struct probes and array elements via the
  referring field, matching the aligned materializer).
- engine read_field/write_field dispatch on the entry's LeafMeta:
  the per-access BastDoc re-parse + lookup_leaf_field walk +
  LeafFieldInfo are gone — the last two review #004 M1 sites.
- Parity note: lookup_leaf_field computed nested-struct defaults from
  the nested struct's own endian annotation; the map now agrees with
  the aligned materializer and packed ReadPlan (referring-field
  propagation). The old divergence (nested struct declaring endian
  under a field that also declares one) is closed; no test pinned it.
- Behavior change: read_field on a map-absent path (whole-struct
  field) errors Offset ("field not found") instead of Access
  ("composite types"); the composite-path test accepted either.
- materialize_aligned's four offset_map.get call sites updated to
  .range.start. alktty/alkcall untouched (bench never uses
  OffsetMap::get; alkcall has no dependency yet).

Verification: 465 tests pass (offset_map tests updated to the
OffsetEntry shape with per-kind LeafMeta expectations; engine test
for the old lookup walk rewritten to assert map entries carry the
LeafMeta); clippy -D warnings clean; cargo doc zero warnings; wasm32
release build green.

docs/plans/030-compiled-forms.md

@@ -526,7 +526,39 @@ warnings`. `cargo build --target wasm32-unknown-unknown --release`.
 
 ---
 
-## Phase 5 — `OffsetMap` carries `LeafMeta` (ADR-012 §2b)
+## Phase 5 — `OffsetMap` carries `LeafMeta` (ADR-012 §2b) — **DONE (2026-09-02)**
+
+> **Status: implemented.** Prerequisite first (review #005 M2):
+> `Hash` added to `Endian`/`VariableEncoding` derives in `schema.rs`
+> (additive; both are fieldless `Eq` enums). New public types
+> `LeafMeta { kind, encoding, endian }` (`Copy + PartialEq + Eq +
+> Hash`) and `OffsetEntry { range, meta }` (with `start()`/`end()`
+> convenience accessors), both re-exported from `lib.rs`; `ByteRange`
+> gained `Hash` (additive). Storage is `Vec<(String, OffsetEntry)>`
+> (deferred decision 2 resolved: struct — `get` returns
+> `Option<&OffsetEntry>`, `iter` yields `(&str, &OffsetEntry)`).
+> `LeafMeta` is computed at `compute` time with **effective** endian
+> threaded through the walk: container default → field override per
+> field, propagated into nested-struct probes and array elements via
+> the referring field (the same propagation the aligned materializer
+> uses). **Parity note:** this replaces `engine.rs`'s
+> `lookup_leaf_field` walk, which computed nested-struct defaults from
+> the *nested struct's own* `endian` annotation — the two paths
+> diverged whenever a nested struct declared `endian` and its
+> referring field also declared one (the map now agrees with the
+> aligned materializer and the packed `ReadPlan`; the old divergence
+> was unreachable through `read_field` only when a nested annotation
+> existed, and no test pinned it). `read_field`/`write_field` dispatch
+> on the entry's `LeafMeta` — the `BastDoc` re-parse +
+> `lookup_leaf_field`/`LeafFieldInfo` per access are gone (the last
+> two M1 sites, engine.rs `read_field`/`write_field`). Behavior
+> change: `read_field` on a path absent from the map (e.g. a
+> whole-struct field path) now errors with `Offset` ("field not found
+> in offset map") instead of `Access` ("does not support composite
+> types") — the composite-path test already accepted either variant.
+> `materialize_aligned`'s four `offset_map.get` call sites updated to
+> `.range.start`. `alktty`/`alkcall` untouched (the bench never uses
+> `OffsetMap::get`; alkcall has no dependency yet).
 
 **Goal:** Extend `OffsetMap`'s entries with `LeafMeta { kind, encoding,
 endian }` computed at `compute` time. `read_field`/`write_field` drop