feat(core,mcp,gui): never apply a legal rule past what wanderbooks knows

The owner (2026-10-13): when what wanderbooks knows is stale, check the
date, research it or ask the owner or CPA, and update the rule for
everyone with a pull request (best) or an issue (worst).

- ledger_core::rules: a registry of rules set by law that change over
  time. Each says the years it's known for, its rules.md entry, its
  vendored official source, and a review date. stale_rules(today) is
  pure; the engine has no clock.
- The 1099 threshold is the first: $600 through 2025, $2,000 in 2026,
  then indexed for inflation (26 U.S.C. §6041(h)). default_threshold
  and vendor_1099_report now return Result; for 2027+ they refuse with
  LedgerError::RuleNotKnown, whose message says what to do: get the
  figure from the official source or the owner/CPA, pass it explicitly,
  then open a PR (or issue) at github.com/swaits/wanderbooks. Before,
  2027+ silently used $2,000.
- MCP: books.vendor_1099 relays that refusal; books.digest gains
  stale_rules; the server instructions tell the agent the procedure (and
  how to handle control-account conflicts: explain, never renumber).
  The CPA package writes a NOT-INCLUDED note instead of a guessed 1099
  worksheet.
- Viewer: "Needs attention" shows each stale rule with a ready prompt
  for the agent; the 1099 page explains instead of guessing.

Review date for the 1099 rule: 2026-11-01, when the IRS usually
publishes next year's inflation adjustments (CONVENTION, marked as such
in the registry).

Tests: an_unknown_year_threshold_is_never_guessed,
dated_rules_go_stale_on_their_review_date (dates passed in, so no clock
in the test), MCP stdio: 2027 refused with guidance, explicit threshold
accepted, digest carries stale_rules (asserted date-independently).

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

docs/spec/rules.md

@@ -289,9 +289,13 @@ years after 2026, indexed to inflation and rounded to the nearest $100
 (§6041(h)). Card payments are reported on Form 1099-K, not 1099-NEC/MISC.
 Attorneys' gross proceeds stay at $600 (not modeled).
 
-**Enforced:** `form1099::default_threshold`. **Open:** for 2027 and later
-the indexed amount is not yet published; the engine must not silently
-assume $2,000 (see the open-questions list in the strategy note).
+**Enforced:** `form1099::default_threshold`, a dated rule
+(`ledger_core::rules::THRESHOLD_1099`, known through 2026, review from
+2026-11-01). For a later year the engine refuses with `RuleNotKnown`
+rather than guess; the MCP digest and the viewer's "Needs attention" list
+flag the rule once it is due (DECISION, the owner, 2026-10-13). **Checked:**
+`an_unknown_year_threshold_is_never_guessed`,
+`dated_rules_go_stale_on_their_review_date`, MCP stdio test.
 
 ## B-20 Every transaction has supporting documents; records are kept