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 the owner): 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. The owner
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

crates/ledger-core/src/reports.rs

@@ -810,8 +810,8 @@ pub struct MembersEquityStatement {
 /// distributions (debits). Earnings are partnership earnings: net income
 /// for the period, and earnings from before it not yet distributed (retained
 /// earnings plus unclosed income), are allocated by share. Allocation is to
-/// the cent with the last member absorbing rounding, so every row's total
-/// is exact and the ending total equals balance-sheet equity.
+/// the cent, odd cents by largest remainder, so every row's total is exact
+/// and the ending total equals balance-sheet equity.
 ///
 /// ```text
 /// GOAL: each member's capital roll-forward for a partnership over a