Release v0.3.0: compiled forms — ReadPlan, owned BastDoc, LeafMeta, ValidationPlan, fingerprinting

Public API bump 0.2.0 -> 0.3.0 (the 030-compiled-forms plan is now
fully implemented; all eight phases landed).

- Cargo.toml: version 0.3.0. lib.rs re-exports complete (ReadPlan +
  sub-types, LeafMeta, OffsetEntry, ValidationPlan + sub-types).
- ADR-007 "Cost" rewritten to the Arc<ReadPlan> cost (15.7 ns) with
  the 0.2.0 "re-parse on demand" framing as a historical note
  (review #004 L2, the last loose end from that review).
- ADR-011/012 status blocks flipped to implemented; architecture
  README ADR table rows updated; layout-engine.md rewritten for the
  0.3.0 surface (engine-factory reader construction, OffsetMap
  OffsetEntry/LeafMeta/fingerprint section, owned BastDoc compute
  signature); SequentialReader module doc points at the engine
  factory. Reviews #004 and #005 flipped to closed.
- Bench re-run (alktty wire_vs_bast, 0.3.0 tree): read p64 98
  ns/chunk (parity with phase 2; hand-rolled 5.7 us/stream),
  layout_build 180 ns (was ~1.2 us — the phase-4 owned-doc cache
  removed the per-build re-parse, ~7x), sequential_reader_new 15.7
  ns, write p64 -3%, engine_compile unchanged (meta-schema
  validation dominates). No dedicated validate_bytes-stream bench:
  the phase-7 spot check (~0.2 us plan-validate vs ~0.6 us
  compile-per-call) stands; a dedicated bench is a follow-up if
  alkcall profiling motivates it.
- Downstream: alktty compiles against the path dep unchanged; alkcall
  has no dependency yet.

Verification (full block, all green): 474 tests; clippy -D warnings
clean; cargo doc zero warnings; wasm32 release build green; cargo
publish --dry-run clean at 0.3.0.

src/sequential_reader.rs

@@ -119,7 +119,8 @@ pub enum FieldValue<'a> {
 /// fields 0..N-1 first. This is inherent to packed layouts where
 /// variable-length fields shift subsequent fields.
 ///
-/// Construct with [`SequentialReader::new`], then drive with
+/// Construct via [`crate::AlkTypeEngine::sequential_reader`] (shares
+/// the engine's compiled plan), then drive with
 /// [`SequentialReader::read_next`] until it returns `Ok(None)`. Use
 /// [`SequentialReader::reset`] to walk the same buffer again, or
 /// [`SequentialReader::read_field`] to seek a single field by name