Implement ReadPlan type + compile (ADR-011 step 1, plan phase 1)

Pure addition: the packed read-side compiled form (src/read_plan.rs)
and lib.rs wiring (module + re-exports of ReadPlan, FieldPlan,
CompositePlan, ReadKind, DiscriminatorPlan). No existing engine code
touched — phases 2-5 wire the plan into the reader/materializer/engine.

- Refined union shape (ADR-011 as refined by review #005):
  CompositePlan::Union { disc, shared, variants } with
  shared: Option<Box<ReadPlan>> for field-disc unions and
  variants: Vec<(String, CompositePlan)> — no VariantPlan/VariantKind,
  nested-union variants work by ordinary CompositePlan recursion
  (restores the 0.2.0 capability the POC rejected).
- by_name is BTreeMap (ADR-012 §1 Hash-derive prerequisite).
- True array strides (deferred decision 4): fixed struct/nested-array
  elements compute their real stride via fixed_composite_size;
  variable-length elements stay 0. 0.2.0 returned 0 for fixed struct
  arrays; that behavioral change rides the 0.3.0 bump (phase 2 will
  surface it through SequentialReader).
- Endianness: effective endian baked at every node. Parity lock: the
  plan propagates the referring field's effective endian into nested
  structs/unions — what the 0.2.0 packed reader/materializer actually
  do — and ignores nested containers' own endian annotations (the POC
  baked s.endian() there; latent divergence, never exercised by its
  equivalence tests). Nested-annotation tests lock this in.
- Untrusted input: compile carries its own depth cap (128) +
  definition-level cycle set (mirrors ValidationPlan::compile), so
  standalone compile is safe on adversarial docs: cyclic refs, deep
  chains, dangling refs, non-struct roots, and non-struct/union
  variants all surface as AlkTypeError::Schema, never a panic.
  Overflow-safe stride arithmetic (checked_mul).

Verification: 388 tests pass (355 existing + 33 new: every BastType
arm coverage, field-disc shared/nested-union compile shape, stride
computation, endian parity, cycle/depth/malformed rejection,
Send + Sync static-bound assertion); clippy -D warnings clean;
cargo doc zero warnings; wasm32-unknown-unknown release build green.

Next: phase 2 (SequentialReader + materialize_packed consume the plan).

docs/plans/030-compiled-forms.md

@@ -1,7 +1,7 @@
 ---
 status: in-progress
 created: 2026-08-19
-last_updated: 2026-08-31
+last_updated: 2026-09-02
 adr: ADR-011, ADR-012
 ---
 
@@ -113,14 +113,26 @@ accessors (signatures), `FieldValue`, `AlkTypeKind`, `AlkTypeError`,
    bug, the stride is behaviorally observable, and we're bumping. The
    plan step calls this out explicitly. Decided in phase 2.
 
-## Phase 1 — `ReadPlan` type + `compile` (ADR-011 step 1)
+## Phase 1 — `ReadPlan` type + `compile` (ADR-011 step 1) — **DONE (2026-09-02)**
+
+> **Status: implemented.** `src/read_plan.rs` builds the refined
+> `CompositePlan::Union` shape (`shared: Option<Box<ReadPlan>>` +
+> `variants: Vec<(String, CompositePlan)>`, no `VariantPlan`/`VariantKind`)
+> with eager `$ref` resolution, `BTreeMap` `by_name`, field-disc `shared`
+> sub-plans, nested-union variant support, and true array strides
+> (deferred decision 4 resolved: fixed struct arrays compute their real
+> stride, not the 0.2.0 reader's `0`). Two parity notes recorded as
+> compile-behavior locks in tests: (a) the plan propagates the
+> *referring field's* effective endianness into nested structs/unions —
+> exactly what the 0.2.0 packed reader/materializer do — rather than
+> consulting nested containers' own `endian` annotations (the POC baked
+> `s.endian()` there; its equivalence tests never covered a nested
+> annotation, so the divergence was latent); (b) `compile` carries its
+> own depth cap (128) + definition-level cycle set, so standalone
+> `ReadPlan::compile` is untrusted-input-safe independent of the
+> meta-schema and the engine's `ValidationPlan` gate.
 
 **Goal:** Add the `ReadPlan`/`FieldPlan`/`CompositePlan`/`ReadKind`/
-`DiscriminatorPlan` types and `ReadPlan::compile(&Value, &str) ->
-Result<Self, AlkTypeError>`. Pure addition; no existing code touched.
-This is the foundation — later phases wire it into the engine and
-reader. (The POC's `VariantPlan`/`VariantKind` types are **dropped**
-in the production shape — see the union-shape note below.)
 
 **ADR reference:** [ADR-011 §The `ReadPlan` shape](../architecture/decisions/011-compiled-read-plan-for-packed-mode.md#the-readplan-shape),
 [ADR-011 §Construction](../architecture/decisions/011-compiled-read-plan-for-packed-mode.md#construction).