fix(kalshi): stabilize event description template and outcome labels

[0xharryriddle]

Problem

Two Kalshi normalization regressions:

1. UnifiedEvent.description could become malformed (e.g. If {x}0, 2026...) when one market in an event had a slightly different suffix date.
2. MarketOutcome.label could become :: Democratic / Not :: Democratic because subtitle was preferred over yes_sub_title for some markets.

Root Cause

• Description logic in core/src/exchanges/kalshi/normalizer.ts used raw longest common prefix/suffix slicing, which is brittle under small suffix drift.
• Label logic in the same file chose market.subtitle || market.yes_sub_title, but Kalshi sometimes uses subtitle for structural metadata (:: Party) rather than candidate names.

What Changed

1. core/src/exchanges/kalshi/normalizer.ts

• Added deriveOutcomeLabel() and cleanLabel():
  • Prefer yes_sub_title.
  • Ignore structural labels starting with ::.
• Replaced old deriveEventDescription() prefix/suffix algorithm with template voting:
  • Template each rules_primary by substituting candidate name with {x}.
  • Choose the most frequent template.
  • Fallback to first rule when needed.
• Prevents malformed template outputs and preserves meaningful event descriptions.

2. core/src/exchanges/kalshi/utils.ts

• Applied the same label precedence/sanitization in the legacy utility mapping path for consistency.

3. core/test/unit/normalizers/kalshi.test.ts

• Added regression tests for:
  • Mixed suffix/date description case (KXCABOUT-26MAR-style).
  • subtitle=':: ...' contamination case (KXGOVCA-26-style).

Validation

• Ran: pnpm -C core test -- kalshi.test.ts

• Result:

> jest -c jest.config.js -- kalshi.test.ts

 PASS  src/exchanges/kalshi/kalshi.test.ts
 PASS  test/unit/normalizers/kalshi.test.ts

Test Suites: 2 passed, 2 total
Tests:       26 passed, 26 total
Snapshots:   0 total
Time:        0.385 s, estimated 1 s
Ran all test suites matching kalshi.test.ts.