Add a 15-second full-depth token repricing horizon

[proerror77]

Parent

#314

Change contract

Add 15-second Up and Down full-depth repricing labels, fillability, slippage, PnL targets, evaluator reports, and search-target registration alongside the existing 5, 10, and 30-second evidence.

Acceptance evidence

• Both token sides expose 15-second bid-change, fillable, full-depth value, slippage, and PnL fields.
• Labels use only future book states at or after the causal 15-second boundary.
• Missing future depth produces unavailable evidence rather than zero PnL.
• Fees, stake size, depth consumption, and sampling limitations match the existing 5, 10, and 30-second contracts.
• AutoFactor and Markov/search surfaces can select 15 seconds without merging it with another horizon.
• Existing 5, 10, and 30-second outputs remain byte or schema compatible unless an explicit version bump is recorded.
• Counterexamples cover look-ahead, wrong token, stale book, and no-fill cases.

Out of scope

New event durations such as 15-minute or 1-hour markets; collector changes; settlement prediction; live execution.

Dependency and merge order

Depends on #230 so the side-bound token execution task identity exists before the 15-second horizon is admitted. This is one horizon addition and one independently revertible PR.

Focused validation

Use TDD at the full-depth label builder, target registry, evaluator artifact, and search allowlist.

Rollout and rollback impact

Research-only additive horizon. Rollback removes 15-second task admission while preserving existing horizons.