Fingerprint ReadPlan/OffsetMap: Hash + Eq + fingerprint() (ADR-012 §1/§4, plan phase 6)

- #[derive(Hash, Eq)] on ReadPlan, FieldPlan, CompositePlan, ReadKind,
  DiscriminatorPlan (schema: Arc<Value> hashes via serde_json Value
  Hash + Eq under preserve_order), and on OffsetMap (+ Clone;
  LeafMeta/OffsetEntry/ByteRange payload already Hash from phase 5 /
  this phase).
- fingerprint() -> u64 on both via std DefaultHasher (deferred
  decision 3 resolved: no new dep, not hot, cross-version stability a
  non-goal per ADR-012).
- Contract tests both sides: equal schemas -> equal PartialEq +
  fingerprint; field-kind / field-order / endianness changes each
  break equality and fingerprint; different root names over the same
  document fingerprint differently (ReadPlan).
- ValidationPlan already carries its own Hash/Eq/fingerprint +
  contract test (phase 7 landed early).

Verification: 474 tests pass (9 new fingerprint contract tests);
clippy -D warnings clean; cargo doc zero warnings; wasm32 release
build green.

docs/plans/030-compiled-forms.md

@@ -641,7 +641,24 @@ wasm32-unknown-unknown --release`.
 
 ---
 
-## Phase 6 — Fingerprinting `ReadPlan`/`OffsetMap` (ADR-012 §1, §4)
+## Phase 6 — Fingerprinting `ReadPlan`/`OffsetMap` (ADR-012 §1, §4) — **DONE (2026-09-02)**
+
+> **Status: implemented.** `#[derive(Hash, Eq)]` added to `ReadPlan`,
+> `FieldPlan`, `CompositePlan`, `ReadKind`, `DiscriminatorPlan`
+> (`ReadPlan`'s `schema: Arc<Value>` hashes fine — `serde_json::Value:
+> Hash + Eq` under the pinned `preserve_order` serde_json) and to
+> `OffsetMap` (`Clone` added alongside; its `LeafMeta`/`OffsetEntry`/
+> `ByteRange` payload gained `Hash` in phase 5 / this phase). The
+> POC's `VariantPlan`/`VariantKind` don't exist in the production
+> shape (phase 1 dropped them). `fingerprint() -> u64` on both via
+> `DefaultHasher` (deferred decision 3 resolved: std `DefaultHasher`,
+> no new dep; the fingerprint isn't hot; cross-version stability is a
+> non-goal per ADR-012). Fingerprint contract tests on both: same
+> schema twice → equal `PartialEq` + equal fingerprint; field-kind
+> change, field-order change, and endianness change each → different
+> fingerprints; (ReadPlan) different root names over the same document
+> → different fingerprints. `ValidationPlan` already carries its own
+> `Hash + Eq` + `fingerprint` + contract test (phase 7).
 
 **Goal:** Add `Hash + Eq` derives + `fingerprint() -> u64` to `ReadPlan`
 and `OffsetMap`. Enables cross-run caching, `alkcall` schema handshake,