Sync docs to 18 BAST kinds (Timestamp removal fallout)

- README: 19 -> 18 kinds, drop the timestamp row from the kinds table,
  drop 'timestamp shape' from the validate_bytes constraint list, remove
  the residual 'upcoming alkcall crate' sentence from the crate
  independence section (alkcall exists now), fix two '19 kinds' refs in
  the documentation pointer list and schema-layer link.
- src/data_access.rs: module doc comment still said 'all 19 AlkType
  kinds' -> 18.
- bast-format.md (normative): 19 -> 18 AlkTypeKind enum variants.
- layout-engine.md: cross-reference to 'the 19 AlkType kinds' -> 18.
- ADR-BAST (bast-bast-format.md): the Decision section claimed the post-
  pivot enum has '19 unchanged' variants; now 18, with the wording
  adjusted so it no longer says 'unchanged' across the pivot.
- ADR-VAL-SPLIT: drop 'timestamp shape' from the value-domain constraint
  list and the validator-arm table row (validate_timestamp no longer
  exists).

Left as historically accurate (describe the v0.1.0 pre-pivot state):
ADR-003/004/006 Context mentions of AlkType:Timestamp, ADR-005 'engine
now has 19', and the '19 jsonschema::Keyword factories' references in
the What-is-removed sections of ADR-BAST and ADR-VAL-SPLIT.

Verification: cargo test --release (407 pass), cargo clippy --all-targets
-- -D warnings (clean), cargo doc --no-deps (clean), cargo build --target
wasm32-unknown-unknown --release (clean).

docs/architecture/bast-format.md

@@ -39,7 +39,7 @@ custom-keyword content.
 3. **`kind`-based vocabulary.** Every type has a `kind` field whose
    value is a known string (`"uint32"`, `"struct"`, `"union"`, etc.).
    This replaces the `AlkType:*` custom-keyword pattern with a flat,
-   easily-matched string. The 19 `AlkTypeKind` enum variants are
+   easily-matched string. The 18 `AlkTypeKind` enum variants are
    unchanged; `AlkTypeKind::from_str`/`to_str` map between the enum and
    the lowercase BAST strings (D-BAST-002).
 

docs/architecture/decisions/bast-bast-format.md

@@ -90,8 +90,8 @@ its consequences; the spec records the shape.
 3. **`kind`-based vocabulary.** Every type has a `kind` field whose
    value is a known string (`"uint32"`, `"struct"`, `"union"`, etc.).
    This replaces the `AlkType:*` custom-keyword pattern with a flat,
-   easily-matched string. The 19 `AlkTypeKind` enum variants are
-   unchanged; `AlkTypeKind::from_bast_str`/`to_bast_str` map between
+   easily-matched string. The 18 `AlkTypeKind` enum variants are
+   the BAST kinds; `AlkTypeKind::from_bast_str`/`to_bast_str` map between
    the enum and the lowercase BAST strings (D-BAST-002).
 4. **Order is explicit.** Struct fields are an ordered array, not an
    object with `properties`. Field order is unambiguous — no reliance

docs/architecture/decisions/val-split-two-validator-model.md

@@ -27,7 +27,7 @@ paths:
    needed for `validate_bytes`; the BAST document *is* the validation
    spec for bytes. The natural validator is a recursive walker over the
    BAST type tree that checks the value-domain constraints the
-   materializer does not (integer ranges, `maxLength`, timestamp shape,
+   materializer does not (integer ranges, `maxLength`,
    enum index bounds, union variant constraints). The POC
    (`bast-validator-poc` branch, `src/bast_poc.rs`) proved this out
    end-to-end with 20 reference tests.
@@ -75,7 +75,6 @@ alone:
 | Float finiteness (Float32/64) | `validate_float` |
 | String `maxLength` (byte length) | `check_string` |
 | Bytes `maxLength` (array length) | `check_bytes` (accepts `Value::String` and `Value::Array`) |
-| RFC 3339 timestamp shape | `validate_timestamp` (non-strict, matching v0.1.0) |
 | Enum index bounds | `validate_enum` — **fixes the v0.1.0 dead constraint** |
 | Union variant dispatch | `validate_union` reads `__discriminator`, resolves the variant, recurses |
 | Struct fields | `validate_struct` walks `fields`, requires each declared field present, recurses |

docs/architecture/layout-engine.md

@@ -356,7 +356,7 @@ See [open-questions.md](open-questions.md) for full details.
   the two layout modes decision
 - [ADR-003](decisions/003-schema-annotations.md) — schema
   annotations
-- [schema-layer.md](schema-layer.md) — the 19 AlkType kinds and their
+- [schema-layer.md](schema-layer.md) — the 18 AlkType kinds and their
   byte sizes
 - [data-access.md](data-access.md) — read/write functions that use the
   computed offsets