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

crates/ledger-core/src/rules.rs

@@ -0,0 +1,79 @@
+//! Rules set by law that change over time, and how far wanderbooks knows
+//! them.
+//!
+//! The engine has no clock, so it never decides on its own that a rule is
+//! out of date. Instead each dated rule says which years it is known for
+//! and when it is due for review; callers that know today's date (the MCP
+//! server, the viewer) ask [`stale_rules`] and surface what is due.
+//!
+//! ```text
+//! GOAL: never apply a legal amount for a year it isn't known for, and say
+//!       so before anyone needs it.
+//! SOURCE: DECISION (owner, 2026-10-13): when what wanderbooks knows is
+//!         stale, research it or ask the owner or CPA, and update the rule
+//!         for everyone (a pull request, or at least an issue, on
+//!         github.com/swaits/wanderbooks).
+//! INVARIANTS:
+//!   1. NEVER returns a rule's value for a year past `known_through`.
+//!   2. MUST report a rule as stale from its review date on.
+//! ```
+//!
+//! To update a rule: vendor the new official text beside the old in its
+//! `source` file (docs/spec/README.md, "Vendoring rules"), extend the
+//! rule's table and `known_through`, move `review_after` to when the next
+//! change is expected, and say so in the commit.
+
+use chrono::NaiveDate;
+
+/// A rule set by law whose value changes over time.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct DatedRule {
+    /// Plain name, e.g. "1099 reporting threshold".
+    pub name: &'static str,
+    /// Its entry in docs/spec/rules.md, e.g. "B-19".
+    pub catalog: &'static str,
+    /// The vendored official text it comes from.
+    pub source: &'static str,
+    /// The last year wanderbooks knows the rule for.
+    pub known_through: i32,
+    /// When to look again, as (year, month, day): the day the next value
+    /// is expected to be published or to take effect.
+    pub review: (i32, u32, u32),
+    /// Why that date, and where the next value will come from.
+    pub review_why: &'static str,
+}
+
+impl DatedRule {
+    /// The review date.
+    pub fn review_after(&self) -> NaiveDate {
+        let (y, m, d) = self.review;
+        NaiveDate::from_ymd_opt(y, m, d).expect("rule review dates are valid")
+    }
+
+    /// Whether the rule is due for review on `today`.
+    pub fn is_stale(&self, today: NaiveDate) -> bool {
+        today >= self.review_after()
+    }
+}
+
+/// The 1099-NEC / 1099-MISC reporting threshold (rules.md B-19): $600 for
+/// payments through 2025, $2,000 in 2026, then indexed for inflation.
+pub const THRESHOLD_1099: DatedRule = DatedRule {
+    name: "1099 reporting threshold",
+    catalog: "B-19",
+    source: "docs/spec/sources/irs-1099-thresholds.md",
+    known_through: 2026,
+    review: (2026, 11, 1),
+    review_why: "26 U.S.C. §6041(h) indexes the threshold for calendar years after 2026, \
+                 rounded to the nearest $100; the IRS publishes each year's inflation \
+                 adjustments late in the year before (CONVENTION: the usual timing, not a \
+                 cited date), so look for the 2027 figure from November 2026.",
+};
+
+/// Every dated rule the engine applies.
+pub const DATED_RULES: &[DatedRule] = &[THRESHOLD_1099];
+
+/// The dated rules due for review on `today`.
+pub fn stale_rules(today: NaiveDate) -> Vec<&'static DatedRule> {
+    DATED_RULES.iter().filter(|r| r.is_stale(today)).collect()
+}