Sync architecture docs and ADRs to BAST pivot (steps 9-10)

Step 9 (convert tests to BAST format) was a no-op: steps 4-8 converted
the tests as they went. The only remaining  reference in
src/tests was the intentional  rejection test at
src/schema.rs:462 (asserting the old keyword form is rejected). Full
suite passes: 389 tests (312 lib + 77 integration).

Step 10 (sync architecture docs and ADRs):

Descriptive docs rewritten/updated for BAST:
- schema-layer.md: rewritten for the BAST parser (BastDoc/BastDef/
  BastType typed tree, AlkTypeKind enum with to_bast_str/from_bast_str,
  what was removed). Points at bast-format.md for the normative format.
- validation.md: rewritten for the two-validator model
  (bast_validation for validate_bytes, standard jsonschema for
  validate_json). Documents the repurposed build_validator, the
  AlkTypeError::Validation uniform payload (D-BAST-009), and what is
  removed.
- builder.md: updated all output examples to BAST JSON
  (struct_() -> { kind: struct, fields: [...] }; object() -> standard
  JSON Schema). Documents build_doc, count(), and the field-name union
  fields requirement (D-BAST-005).
- overview.md: updated for BAST (what/why, schema-is-the-format table,
  dependencies, architecture pointers, design decisions table).
- README.md (architecture index): updated document table, ADR table
  (new ADR-BAST + ADR-VAL-SPLIT, superseded ADR-001), OQ table
  (OQ-007/OQ-008 resolutions updated for BAST-native validator), and
  key design principles (#1, #2, #7, #10 reworded for BAST).
- data-access.md: updated tunion function signatures to BastUnion and
  the variant resolution to return BastType (resolve_typeref for refs).
- layout-engine.md: updated construct signatures
  (LayoutBuilder::new(bast_doc, root_name), OffsetMap::compute(&doc),
  SequentialReader::new(bast_doc, root_name)), the recursive-walk
  description (BAST typed tree), and composite-kind headings
  (TStruct/TUnion/TArray -> struct/union/array). Added D-BAST-004
  note on array count requirement.

New ADRs:
- ADR-BAST (bast-bast-format.md): the BAST format, meta-schema,
  //kind vocabulary, design principles, what is removed, the
  enum index bounds bug fix. Supersedes ADR-001's format-specific
  content; records D-BAST-001..009.
- ADR-VAL-SPLIT (val-split-two-validator-model.md): the two-validator
  model (BAST-native for validate_bytes, standard jsonschema for
  validate_json), the repurposed build_validator, the uniform
  AlkTypeError::Validation payload. Refines ADR-004's validation
  strategy and ADR-010's validation step; records D-BAST-006/007/009.

Amended ADRs (supersession/amendment notes added; original decision
text preserved as historical record):
- ADR-001: format-specific content superseded by ADR-BAST;
  purpose/scope and schema-is-the-format principle retained.
- ADR-002: unchanged under the pivot; one-line note that the input
  format changed but the modes didn't.
- ADR-003: annotation semantics retained; annotation location moved
  to BAST type-level properties (amended by ADR-BAST).
- ADR-004: AlkTypeError enum retained (D-BAST-009); validation
  strategy section refined by ADR-VAL-SPLIT.
- ADR-009: builder API surface retained; build() output format
  amended to BAST / standard JSON Schema by ADR-BAST (D-BAST-008).
- ADR-010: validate_bytes two-step concept retained; validation step
  amended to the BAST-native validator by ADR-VAL-SPLIT.

Other:
- Cargo.toml description: JSON Schema with AlkType:* custom keywords
  -> BAST document.
- bast-pivot.md research record: status draft -> implemented, with a
  pointer to the ADRs that superseded its decisions.
- bast-implementation.md plan: status draft -> complete, with a note
  that step 9 was a no-op and step 10 is this commit.
- open-questions.md: OQ-006/OQ-007/OQ-008 resolutions updated for the
  BAST-native validator.
- questions/008-unionvalidator-variant-dispatch.md: added a
  post-BAST-pivot note pointing to the current bast_validation
  implementation; v0.1.0 resolution text preserved as historical
  record.

Verification:
- cargo test --release: 389 pass (312 lib + 77 integration)
- cargo clippy --all-targets -- -D warnings: clean
- cargo doc --no-deps: clean
- cross-reference check: every relative link in the new/updated docs
  resolves (verified by script).

docs/research/bast-pivot.md

@@ -1,11 +1,32 @@
 ---
-status: draft
+status: implemented
 created: 2026-08-14
 last_updated: 2026-08-15
 ---
 
 # BAST Pivot — Research Record
 
+**Status: implemented.** The BAST pivot landed in steps 1–10 of the
+[implementation plan](../plans/bast-implementation.md) (commits
+`66ab9d7` → `54fd112` on `origin/main`). The decisions D-BAST-001..009
+are recorded in two new ADRs —
+[ADR-BAST](../architecture/decisions/bast-bast-format.md) (the format)
+and [ADR-VAL-SPLIT](../architecture/decisions/val-split-two-validator-model.md)
+(the two-validator model) — which supersede the format-specific content
+of [ADR-001](../architecture/decisions/001-alktype-purpose-scope-jsonschema-engine.md)
+and refine the validation strategy of
+[ADR-004](../architecture/decisions/004-error-handling-validation-strategy.md)
+and [ADR-010](../architecture/decisions/010-generalized-validation-validate-bytes.md).
+The normative format spec is
+[`docs/architecture/bast-format.md`](../architecture/bast-format.md);
+the parser is documented in
+[`docs/architecture/schema-layer.md`](../architecture/schema-layer.md).
+
+What follows is the original research record — the *why* and *what was
+proved*, preserved as the historical grounding for the decisions.
+
+---
+
 Replace alktype's custom JSON Schema keywords (`AlkType:Uint32`,
 `AlkType:Struct`, etc.) with a standalone JSON format — BAST (Binary
 Abstract Syntax Tree) — that describes binary data layouts using a