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.

checker/tests/docs.rs

@@ -0,0 +1,28 @@
+//! The documentation bookspec's identifiers live in: `docs/spec/rules.md`
+//! carries each stated rule's id and speaks bookspec's `CHOICE`.
+
+use std::path::Path;
+
+use bookspec_checker::rules;
+
+/// `docs/spec/rules.md` carries every stated rule's id under its heading,
+/// and no longer calls a choice a decision (P13 step 3).
+#[test]
+fn rules_md_carries_every_id() {
+    let text = std::fs::read_to_string(
+        Path::new(env!("CARGO_MANIFEST_DIR")).join("../../docs/spec/rules.md"),
+    )
+    .expect("docs/spec/rules.md");
+    for rule in rules().iter().filter(|r| r.status.is_some()) {
+        assert!(
+            text.contains(&format!("**bookspec:** `{}`", rule.id)),
+            "B-{} carries {}",
+            rule.b,
+            rule.id
+        );
+    }
+    assert!(
+        !text.contains("DECISION"),
+        "rules.md speaks bookspec's CHOICE"
+    );
+}