feat(bookspec): rule identifiers from B-1 to B-45 (P13)

Every rule of `docs/spec/rules.md` now has one bookspec identifier,
`BS-<area>-<n>`: eleven areas (entry, ledger, statement, period, partner,
asset, prepaid, arap, revenue, 1099, format), numbered in B-rule order
within its area. The table is `bookspec/spec/rules-index.md`, and each
stated rule carries its id under its heading in `docs/spec/rules.md`, so
a reader of either can find the other.

rules.md's DECISION status becomes bookspec's CHOICE: the specification
names the options and an engine declares its pick (`Claims::with_choice`),
which is what an example already meant by `status = "CHOICE"`. B-39 to
B-45 are allocated in `plan/served.md` 8.0.3 and not yet stated: they
carry an id and an area and no status, so a later unit fills them in
without moving anyone else's number.

The catalogue names the examples that demonstrate each rule (56 files,
each under at least one rule) and the rule each refusal reason belongs
to. A rule with no example cannot be claimed: a claim needs evidence.
`docs/architecture.md`, `docs/testing.md` (tests and the For-P12
mutation targets) and `docs/parity.md` record the new shape; the
bookspec README points at the checker, the runner and the adapter.

docs/parity.md

@@ -71,3 +71,13 @@ do everything the GSheet+GDrive system does today? Tracked separately in
 `docs/capability-parity.md`, with its own cutover rule (parallel run +
 the owner's approval). The two catalogs converge: when both are green, the
 sheet retires.
+
+### P13 bookspec (2026-10-26)
+
+P13 is a specification, not a feature, so no row of the semantic catalog
+moves and the floor stands. It adds the checker crate (`bookspec/checker`:
+the example schema, the rule identifiers `BS-<area>-<n>` and the `Engine`
+trait), the runner (`bookspec/runner`: pass, fail or not claimed per rule),
+and the rule index from B-1 to B-45 (`bookspec/spec/rules-index.md`). Every
+existing bookspec example still runs on every `cargo test`, through both
+the loader's own checks and the `Engine` trait.