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.

README.md

@@ -13,6 +13,14 @@ Every example is checked against wanderbooks on every `cargo test`
 with fails the build, so the examples can't rot. An expected value is worked
 by hand from the cited source, never copied from an engine's output.
 
+The schema, the parser (`Example::from_toml`, every structure with
+`deny_unknown_fields`) and the `Engine` trait an accounting engine
+implements live in `bookspec/checker`; `bookspec/runner` runs any engine
+through the examples and reports per rule: pass, fail or not claimed. The
+wanderbooks adapter is `crates/ledger-core/tests/bookspec.rs`. The rule
+identifiers are `BS-<area>-<n>`, tabled in `bookspec/spec/rules-index.md`
+(P13 steps 1 to 3).
+
 ## The shape of an example
 
 One TOML file per example in `examples/`, named for its `id`.
@@ -99,7 +107,7 @@ declares `largest-remainder`, the rule a members close splits by.
 authorities alone (a standard, a regulation, a statute, a court, the
 mechanics of double entry); `CONVENTION`, common practice where no
 authority governs (rules.md's CONVENTION); `CHOICE`, an owner's or the
-house's decision (a plan D-number, or a DECISION rules.md records) that
+house's decision (a plan D-number, or a CHOICE rules.md records) that
 some expected value depends on, even when authorities carry the rest. An
 example whose figures follow a decision cites it in `sources`.