docs(core): written specs, with sources, for the commands and reports

Max-testing pass, step 2: every domain decision in ledger.rs and
reports.rs gets a GOAL / SOURCE / INVARIANTS / SCENARIOS block in its
doc comment: post_entry, try_create_entry, close_period_to,
mark_cleared, reconciliation_report, account_balance, create_invoice,
create_bill, receive_payment, pay_bill, register_asset, dispose_asset,
run_payroll, remit_payroll_taxes, set_line_class, set_entry_vendor;
trial_balance(_as_of), profit_and_loss_for_period, balance_sheet_as_of,
general_ledger, members_equity, cash_flow, profit_and_loss_by_class,
snapshot. Sources trace to docs/spec/rules.md (and through it to
Pacioli, the IRS, FASB); what no document settles is marked CONVENTION,
DECISION, or ASSUMED for the owner to confirm. The specs are the input to
spec-blind test writing (the next step): tests derived from the spec
alone, never from the code.

Also: rustdoc now runs in `just check` with -D warnings; fixed the seven
existing broken intra-doc links and one bare URL.

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

crates/ledger-core/src/lib.rs

@@ -12,7 +12,7 @@
 //! * All mutations go through [`Ledger`] commands (`create_entry`, `add_line`,
 //!   `post_entry`, `reverse_entry`). Posted entries are immutable; corrections
 //!   are reversing entries.
-//! * Reports ([`trial_balance`], [`balance_sheet`], [`profit_and_loss`]) are
+//! * Reports ([`trial_balance`](reports::trial_balance), [`balance_sheet`](reports::balance_sheet), [`profit_and_loss`](reports::profit_and_loss)) are
 //!   pure functions of a ledger, which is what makes the semantic oracle
 //!   (`docs/oracle.md`) possible.