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

Stephen (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

crates/ledger-core/src/error.rs

@@ -102,6 +102,27 @@ pub enum LedgerError {
         placeholder: bool,
     },
 
+    /// A rule set by law (a dollar threshold, a rate) isn't known to
+    /// wanderbooks for the year asked; the engine never guesses (see
+    /// [`crate::rules`]).
+    #[error(
+        "wanderbooks knows the {rule} only through {known_through}, not for {year}. \
+         Find the {year} figure from the official source (see {spec}) or ask the books' \
+         owner or CPA, and pass it explicitly. Then update the rule for everyone: open a \
+         pull request (or at least an issue) at https://github.com/swaits/wanderbooks with \
+         the new value and its official source."
+    )]
+    RuleNotKnown {
+        /// The rule's plain name.
+        rule: String,
+        /// The year asked for.
+        year: i32,
+        /// The last year the rule is known for.
+        known_through: i32,
+        /// The vendored official source of the rule.
+        spec: String,
+    },
+
     /// The referenced journal entry does not exist.
     #[error("entry {0} does not exist")]
     UnknownEntry(u64),