Resolve review #005: refine 0.3.0 plan + ADR-011/012

Resolve all 11 findings from the 0.3.0 plan review (#005) in one
docs-only pass. No source changes; the crate still builds/tests at
v0.2.0. The one substantive decision change is M3 (per user
direction: ship ValidationPlan in 0.3.0, no more hedging); the rest
are spec corrections or pre-implementation refinements to types that
do not yet exist on main.

- H1: refine ADR-011 CompositePlan::Union to carry
  shared: Option<Box<ReadPlan>> (field-disc shared fields) and
  variants: Vec<(String, CompositePlan)> (drop VariantPlan/
  VariantKind). Plan phase 1 implements the refined shape.
- H2: plan phase 2 specifies ReadPlan stores schema: Arc<Value>
  (not &Value), avoiding the self-referential struct ADR-011
  rejects. Verified serde_json::Value: Hash + Eq holds with
  preserve_order, so phase 6 derives are not blocked.
- M1: nested-union support falls out of the H1 shape refinement
  (a variant can be CompositePlan::Union) — option (a) from the
  review, no behavioral drop vs 0.2.0, no Semver regression row.
- M2: plan phase 5 adds an explicit first sub-step to derive Hash
  on Endian and VariableEncoding in src/schema.rs (additive,
  semver-safe prerequisite the original plan omitted).
- M3: reverse the ValidationPlan deferral. ADR-012's "Deferring
  ValidationPlan" becomes "ValidationPlan — in scope for 0.3.0";
  new ADR-012 §3 commits the decision (compiled form, no per-buffer
  BastDoc walk, Hash + Eq + fingerprint()) and defers only the
  concrete shape to a follow-on design session + the plan's new
  phase 7. Plan gains phase 7 (ValidationPlan); old phase 7 (bump)
  renumbered to phase 8. ADR-011's Out-of-scope and Scope
  Boundaries bullets updated to point at ADR-012 §3. The deferral
  black hole this review's methodology flagged is closed: the work
  is committed with a concrete reactivation trigger, not hedged
  into an unplanned future.
- L1: plan phase 2 corrects the dummy_field_for/ty_source removal
  claim — only packed-side call sites go away; the helpers stay
  for the aligned materialize_leaf_at path.
- L2: plan phase 2 states the packed-vs-aligned
  materialize_typeref_packed split (packed gets a new plan-walking
  function; the existing function stays for aligned).
- L3: plan phase 5 adds a Scope Boundary note — aligned
  materialize's BastDoc structure walk is the permanent 0.3.0
  design; an AlignedPlan is out of scope, tracked as an OQ.
- N1: fix "back-comat" -> "back-compat" typo.
- N2: plan phase 1 verification adds the read_plan_is_send_sync
  static-bound assertion test ADR-011 requires.
- N3: Semver Contract table notes the Result drop on
  SequentialReader::new (Result<Self, AlkTypeError> -> Self)
  alongside the argument-type change.

Also: ADR-012 title -> "Plan Fingerprinting, ValidationPlan, and
Closing the Deferred M1 Sites in 0.3.0"; §3 (Fingerprinting
OffsetMap) renumbered to §4; README ADR table updated; review #005
gets a Resolution section recording how each finding was closed.

Verification (docs-only change, v0.2.0 unchanged):
  cargo test --release                     ok (310 crate + 86 integration + 2 doctests)
  cargo clippy --all-targets -- -D warnings  ok
  cargo doc --no-deps                      ok

docs/architecture/README.md

@@ -46,7 +46,7 @@ format definition; the engine is generic.
 | [009](decisions/009-builder-api.md) | Builder API for Schema Construction | Fluent Rust API producing `serde_json::Value`; covers BAST kinds + standard JSON Schema; resolves OQ-003. *Output format amended to BAST / standard JSON Schema by ADR-BAST.* |
 | [010](decisions/010-generalized-validation-validate-bytes.md) | Generalized Validation — `validate_bytes` on `AlkTypeEngine` | Single-call binary-buffer validation; materialize `Value` from bytes, then validate. *Validation step amended to the BAST-native validator by ADR-VAL-SPLIT.* |
 | [011](decisions/011-compiled-read-plan-for-packed-mode.md) | Compiled Read Plan for Packed Mode | `ReadPlan` — the packed read-side compiled form, symmetric to `OffsetMap` (aligned) and `PackedLayout` (packed write). Closes review #004's 400x read-path gap; retires ADR-007's "re-parse on demand" framing. *Accepted.* |
-| [012](decisions/012-plan-fingerprinting-and-m1-closure.md) | Plan Fingerprinting and Closing the Deferred M1 Sites in 0.3.0 | `ReadPlan`/`OffsetMap` `Hash + Eq` + `fingerprint()`; owned `BastDoc` (lifetime removal); `OffsetMap` carries `LeafMeta` to close the aligned-side M1 sites. Bundles with ADR-011 into one 0.3.0 breaking release. *Proposed.* |
+| [012](decisions/012-plan-fingerprinting-and-m1-closure.md) | Plan Fingerprinting, ValidationPlan, and Closing the Deferred M1 Sites in 0.3.0 | `ReadPlan`/`OffsetMap`/`ValidationPlan` `Hash + Eq` + `fingerprint()`; owned `BastDoc` (lifetime removal); `OffsetMap` carries `LeafMeta` to close the aligned-side M1 sites; `ValidationPlan` retires the interpretive `bast_validation` walk (review #005 M3 reversed the original deferral). Bundles with ADR-011 into one 0.3.0 breaking release. *Proposed.* |
 
 ## Relevant Open Questions