test(core): spec-blind suites and a live GnuCash oracle; four fixes they found

Max-testing pass, steps 2-4.

Spec-blind tests (rank 1-2): four agents, each given only the specs and
the public interface (doc comments and signatures, every body removed,
never src/ or the existing tests), wrote tests/spec_reports.rs (62),
spec_ledger.rs (72), spec_arap.rs (42), spec_assets_payroll.rs (50):
hand-computed examples per scenario and a proptest law per invariant
(conservation, partition, invariance, scaling, reversal cancels,
"a refusal leaves the books unchanged" over random invalid commands).
They found:

1. A refused payroll run changed the books: an all-zero run passed
   validation, then failed at post after creating seven payroll
   accounts and using up an entry id. validate_payroll_lines now
   refuses a run with no gross pay.
2. A refused add_account used up an account id (empty code, duplicate,
   unknown parent). The id is now taken only when the account is added.
3. Stale prose: close_period_to and members_equity still described the
   old last-member rounding.
4. Open (left #[ignore]d pending Stephen): depreciation_schedule's doc
   says "through the month containing" the date; the code prorates that
   month by days.

GnuCash oracle (rank 3, an oracle never the spec): `just oracle` builds
GnuCash 5.10 in a container pinned by base digest and package versions
(oracle/gnucash/) and runs tests/oracle_gnucash.rs: 200 seeded random
books built through wanderbooks (entries, foreign lines, reversals,
monthly closes), every account's balance at month ends and random dates
and each month's P&L, compared exactly: 15,800 comparisons. First run:
4,724 differences, all from foreign lines; without them, zero.
Adjudicated from principles (rules.md B-3): wanderbooks booked
foreign * rate unrounded ($108.34567); a ledger in one money holds no
fractions of its smallest unit, and its cash must match a bank. Stephen
decided: book in the base currency's minor unit, half away from zero.
add_line_fx now rounds; minor units per ISO 4217 list one (2026-09-17,
SIX), generated into Currency::minor_units and recorded in
docs/spec/sources/iso-4217.md. Replay now takes the journaled base
amount (replay_add_line_fx), so lines booked before replay exactly as
booked (contract: books_written_under_older_rules_still_open). Now:
zero differences, foreign lines included.

Tests: 449, from 223.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

docs/oracle.md

@@ -79,7 +79,15 @@ periods, closing to net income), and a property test bridges the two.
 `just prove`; see `docs/verification.md`. Its first pass found three
 engine bugs that Layers 1–4 had all missed.
 
-**Layer 5 — Cross-implementation diff harness (roadmap phase 3).** A
+**Layer 5 — Cross-implementation diff harness (live: GnuCash).**
+`just oracle` builds GnuCash 5.10 in a pinned container
+(`oracle/gnucash/`) and runs `tests/oracle_gnucash.rs`: 200 random books
+built through wanderbooks (entries, foreign lines, reversals, closes),
+every account's balance at month ends and random dates, and each month's
+P&L, compared exactly (15,800 comparisons). Its first run found one real
+difference, adjudicated from principles: wanderbooks kept sub-cent
+amounts on foreign lines; it now books in cents (rules.md B-3). The
+original plan follows. A
 `ledger-oracle` crate will run fixture books through our engine and through
 other implementations (GnuCash headless first), then diff `snapshot()`
 output automatically: `cargo xtask oracle`. This is the direct analog of