docs(spec): every rule traced to an authority; public-domain sources vendored

Stephen asked that specs come from recognized documents, not from
anyone's memory. There is no single US bookkeeping bible; authority is
layered, and this records it:

- docs/spec/sources/: verbatim excerpts (whitespace normalized), each
  with URL, retrieval date, and the SHA-256 of the fetched file, of
  public-domain texts: Pacioli's 1494 treatise in Geijsbeek's 1914
  translation (double entry, the trial balance, one money, write only
  forward, correct by opposite entry, close into capital); IRS Pub. 583,
  538, 946, 15; the 1099-MISC/NEC instructions (2025 and 2026); 26 U.S.C.
  §6041, §6041A, §704; Treas. Reg. §1.263(a)-1(f) and -4(f); Notice
  2015-82; Rev. Proc. 98-25; Form 1065 instructions; 17 U.S.C. §105. The
  excerpts were extracted by matching each passage against the fetched
  file, so none can contain text the source doesn't.
- docs/spec/rules.md: the rule catalog, B-1 to B-22. Each rule says
  AUTHORITY, CONVENTION, or DECISION; cites its sources (FASB's
  Codification and Concepts Statement 8 and OpenStax quoted briefly,
  never copied: copyright and CC BY-NC-SA); and names the code that
  enforces it and the tests and proofs that check it.
- Spec blocks in the code now carry SOURCE: lines into the catalog;
  AGENTS.md makes sourcing rule 0 of the formal-methods bar.

Corrections the sources turned up: the de minimis limit is $500 in the
regulation and $2,500 by Notice 2015-82 (both now cited); 1099
thresholds after 2026 are inflation-indexed, so the engine's flat
$2,000 for 2027+ is an open question (B-19). Unverified ASC paragraphs
(360-10-35-4, 340-10, 230-10-45-10, 606-10-55-37B, 210) are listed in
docs/spec/README.md until someone with a FASB basic-view login reads
them.

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

crates/ledger-core/src/kernel.rs

@@ -8,21 +8,25 @@
 //! laws it implements hold for books of any size. See
 //! `docs/verification.md` for which proof covers which law.
 //!
-//! The laws, in the order they appear:
+//! The laws, in the order they appear (each traced to its authority in
+//! `docs/spec/rules.md`):
 //!
-//! * a line's signed amount is `+amount` for a debit, `-amount` for a credit;
+//! * a line's signed amount is `+amount` for a debit, `-amount` for a credit
+//!   (B-2);
 //! * an entry is acceptable when it has two or more lines, every amount is
-//!   positive and in range, and its debits equal its credits exactly;
-//! * an acceptable entry nets to zero, account by account summed;
+//!   positive and in range, and its debits equal its credits exactly (B-1,
+//!   B-2, B-21);
+//! * an acceptable entry nets to zero, account by account summed (B-1);
 //! * a reversal flips every side, so it is acceptable exactly when the
-//!   original is, and the two together net every account to zero;
+//!   original is, and the two together net every account to zero (B-9);
 //! * an account's balance is its movement read on its normal side, so the
 //!   balances satisfy the accounting equation whenever movements net to
-//!   zero;
+//!   zero (B-5, B-7);
 //! * a trial balance puts each movement in exactly one column, so its
-//!   columns differ by the net of all movements;
-//! * a closing line zeroes the account it closes;
-//! * an allocation's parts sum to the whole, to the last unit.
+//!   columns differ by the net of all movements (B-6);
+//! * a closing line zeroes the account it closes (B-11);
+//! * an allocation's parts sum to the whole, to the last unit, by largest
+//!   remainder (B-13).
 
 use crate::{AccountType, Side};
 use rust_decimal::Decimal;
@@ -277,6 +281,8 @@ pub fn plug_last<T: Amount>(whole: T, parts: &mut [T]) -> Option<()> {
 /// ```text
 /// GOAL: split a whole into parts, each a whole number of units where
 ///       possible, summing to the whole exactly, without favoring anyone.
+/// SOURCE: docs/spec/rules.md B-13 (shares: 26 U.S.C. §704(a); cent
+///         rounding: DECISION, Stephen 2026-10-13), B-21 (exact totals).
 /// INVARIANTS:
 ///   1. MUST make the parts sum to `whole` exactly.
 ///   2. MUST change each part by zero or one unit (the first in order