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/spec/rules.md

@@ -59,8 +59,17 @@ that every line reads one way (2026-10-13, found by the formal pass).
   functional currency of the recording entity by use of the exchange rate
   in effect at that date."
 
+A foreign line is booked in the base currency's smallest unit (ISO 4217
+minor units, `sources/iso-4217.md`), half away from zero; a ledger in one
+money holds no fractions of its smallest unit, and its cash can then
+match a bank statement: DECISION (Stephen, 2026-10-13), after the GnuCash
+cross-check found wanderbooks kept $108.34567 where GnuCash booked
+$108.35. Lines booked before then replay as booked.
+
 **Enforced:** `Ledger::add_line` (base currency only),
-`Ledger::add_line_fx` (base amount = foreign amount × rate, original kept).
+`Ledger::add_line_fx` (base amount = foreign amount × rate, rounded to
+the minor unit; original kept). **Checked:**
+`a_foreign_line_is_booked_in_whole_base_units`; the GnuCash oracle.
 **Checked:** `foreign_currency_line_is_rejected`, `fx_overflow_is_an_error`.
 
 ## B-4 Five elements, closed list