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/assets.rs

@@ -5,7 +5,7 @@
 //! separate set of books with its own rules and stays with the CPA. This
 //! module never pretends to do tax.
 //!
-//! The register is master data owned by [`Ledger`]: assets are journaled
+//! The register is master data owned by [`Ledger`](crate::Ledger): assets are journaled
 //! like accounts (see `ledger-store`), and ids are allocator-deterministic
 //! for replay. Depreciation itself is pure math — [`depreciation_schedule`]
 //! — and postings go through the normal entry machinery, so every