feat(bookspec): the checker crate, the Engine trait and the runner (P13)

bookspec is a specification first: every rule normative and sourced,
every rule demonstrated by concrete examples, and any accounting engine
checked against them through one interface (D-34). This moves the
examples' schema out of the test that happened to parse them and makes
the interface explicit.

- `bookspec/checker` (its own workspace, inside this repository until
  the harness is stable): `Example`, `Setup`, `Event`, `Expect`, `Query`,
  `Refusal` and the TOML parser, every structure `deny_unknown_fields`,
  with `Example::from_toml` checking the cross-field shape (closed lists,
  exact decimal amounts, dates, one event kind, `reverses` counting from
  1, the shares summing to 1, commands accepted or refused and not both).
- `Engine` (`load`, `apply`, `query`, `claims`) and `Refusal { rule,
  reason }`; the refusal's rule is the bookspec identifier, so a refusal
  names the rule it belongs to. `Claims` also carries the option an
  engine picks for a CHOICE rule.
- `bookspec/runner` drives any `Engine` through the examples and reports
  per rule: pass, fail, not claimed. It compares the answer itself; an
  engine is never asked whether it agrees with itself.
- `crates/ledger-core/tests/bookspec.rs` is the wanderbooks adapter over
  `Store`: the loader parses through the checker, its own checks are
  unchanged, and the new `the_runner_reports_every_claimed_rule_green`
  runs all 56 examples through the trait. The IIF appendix is answered
  through `ledger_package::export_iif`, the code its own test checks.
- `just check` runs the bookspec workspace's fmt, clippy, rustdoc and
  tests too, so an example cannot rot (D-34, Z-17).

The plan's trait has four methods and no path for an example's
`[[changes]]` (a rename, the policy, an asset correction), which post no
entry: `change` is the fifth, required so an engine cannot skip those
examples silently. This is reported to the EM as the one departure.

One money rule holds here too: an answer's figures are exact decimal
strings, never JSON numbers, so no engine can round a cent through a
float.

bookspec/checker/src/lib.rs

@@ -0,0 +1,23 @@
+//! bookspec's checker: the example schema, the rule identifiers, and the
+//! [`Engine`] trait an accounting engine implements (`plan/served.md` P13,
+//! D-34).
+//!
+//! bookspec is a specification first: every rule is stated normatively,
+//! sourced, and demonstrated by concrete examples (`bookspec/examples/`).
+//! This crate is the machine-readable part: [`Example`] is the schema and
+//! its parser, [`mod@rules`] the catalogue (`BS-<area>-<n>`), and [`Engine`]
+//! the only interface a conformance runner needs. The runner lives in
+//! `bookspec-runner`; wanderbooks' adapter is
+//! `crates/ledger-core/tests/bookspec.rs`.
+//!
+//! The crate knows nothing of any engine: no money type, no ledger, no
+//! store. That is what lets a second engine (the GnuCash oracle, later) be
+//! checked by the same examples.
+
+pub mod engine;
+pub mod rules;
+pub mod schema;
+
+pub use engine::{Claims, Engine, Refusal, RuleId, RuleIdError, fallback_refusal};
+pub use rules::{Area, Rule, Status, covered_rules, rule, rule_for_id, rule_for_refusal, rules};
+pub use schema::*;