Decompose BAST pivot doc into normative spec + implementation plan

The bast-pivot.md research doc had grown to 1477 lines (~58KB) through
iterative editing, pushing its most actionable content (D-BAST
decisions, POC result, migration steps) past the 50KB Read tool cap.
Agents peeking at the truncated file landed in duplicated/out-of-order
sections. Decompose into three readable-sized files with distinct roles:

- docs/architecture/bast-format.md (28KB, new): the normative BAST
  format spec -- meta-schema, TypeRef, examples, validation model.
  Grounded in the POC and D-BAST-001..009. Stable and safe to write
  now; schema-layer.md/validation.md stay describing current code and
  are rewritten post-implementation (per AGENTS.md ADR-grounding rule).
- docs/plans/bast-implementation.md (31KB, new): the execution entry
  point -- ordered 10-step plan with per-step goal/files/spec-ref/
  verification, the public-API semver contract table up front as a
  scope-creep guardrail, and the ADR-sync checklist at the end. Each
  step links to the specific bast-format.md section and D-BAST anchor.
- docs/research/bast-pivot.md (28KB, trimmed): now the research record
  only -- Summary, Motivation, POC scope/result, Decisions, Risks,
  References. The normative format spec, what-changes tables,
  validator-split details, and migration steps moved to the two new
  docs; pointers added. 1155 lines removed, 216 added.
- docs/architecture/README.md: index updated to list bast-format.md
  and the two in-progress pivot docs, with notes on schema-layer.md
  and validation.md being rewritten when the pivot lands.

All three files are under the 50KB Read cap, so an implementing agent
gets the whole document in one call. Cross-reference anchors verified
to resolve. No code changes; cargo test --release (396 tests) green.

Verification: cargo test --release (310 crate + 86 integration, all pass).

docs/architecture/README.md

@@ -15,12 +15,20 @@ format definition; the engine is generic.
 | Document | Status | Description |
 |----------|--------|-------------|
 | [overview.md](overview.md) | draft | Crate purpose, "schema is the format" principle, dependencies, consumers, scope boundaries |
-| [schema-layer.md](schema-layer.md) | draft | The 19 `AlkType:*` kinds, jsonschema custom keyword integration, TypeBox interop, schema annotations |
+| [schema-layer.md](schema-layer.md) | draft | The 19 `AlkType:*` kinds, jsonschema custom keyword integration, TypeBox interop, schema annotations. *(Current v0.1.0 format; will be rewritten for BAST — see [bast-format.md](bast-format.md).)* |
+| [bast-format.md](bast-format.md) | draft | **Target schema format.** BAST (Binary Abstract Syntax Tree): meta-schema, TypeRef, examples, the two-validator model. Supersedes the format-spec content of schema-layer.md when the [BAST pivot](../plans/bast-implementation.md) lands. |
 | [layout-engine.md](layout-engine.md) | draft | Offset computation, the two layout modes (packed sequential vs aligned static), alignment, endianness, variable-length handling |
 | [data-access.md](data-access.md) | draft | Read/write functions, TUnion dispatch, field paths, zero-copy access, length-prefix reading |
-| [validation.md](validation.md) | draft | Custom keyword validators for all 19 `AlkType:*` kinds, `AlkTypeError`, load-time vs access-time validation, `AlkTypeEngine`; `validate_bytes` for binary buffers (ADR-010) |
+| [validation.md](validation.md) | draft | Custom keyword validators for all 19 `AlkType:*` kinds, `AlkTypeError`, load-time vs access-time validation, `AlkTypeEngine`; `validate_bytes` for binary buffers (ADR-010). *(Current v0.1.0 validation; will be rewritten for the validator split — see [bast-format.md §Validation Model](bast-format.md#validation-model).)* |
 | [builder.md](builder.md) | draft | Fluent Rust API for constructing alktype JSON Schemas at runtime, producing `serde_json::Value`; covers AlkType kinds + standard JSON Schema (ADR-009) |
 
+### In-progress work
+
+| Document | Status | Description |
+|----------|--------|-------------|
+| [BAST pivot — research record](../research/bast-pivot.md) | draft | Motivation, POC scope and result, decisions D-BAST-001..009, risks for the BAST format pivot |
+| [BAST pivot — implementation plan](../plans/bast-implementation.md) | draft | Ordered implementation steps, the public-API semver contract, and the ADR-sync checklist for the BAST pivot |
+
 ## Applicable ADRs
 
 | ADR | Title | Relevance |