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/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