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/ledger.rs

@@ -612,6 +612,8 @@ impl Ledger {
     ///
     /// ```text
     /// GOAL: decide whether `entry` may be reversed on `date`.
+    /// SOURCE: docs/spec/rules.md B-9 (Pacioli ch. 31, 36), B-10 (DECISION,
+    ///         Stephen 2026-10-13), B-12 (closed periods).
     /// INVARIANTS:
     ///   1. MUST NOT mutate anything.
     ///   2. NEVER accepts a reversal that `reverse_entry` would then refuse.