feat: expand corporate actions and instrument lifecycle

[fallblu]

Summary

• add exact stock-dividend, rights, and spin-off accounting with explicit basis and fractional-entitlement policies
• add stable instrument lifecycle state for halts, resumes, identifier changes, expiration, and delisting
• introduce scenario/journal contract v12 and strategy protocol v10 while preserving frozen v11/v9 compatibility
• audit lifecycle causality, order cancellation, target adjustment, and terminal cash-out behavior

Relates to #31.
Relates to #32.

Test plan

• make check
• make coverage (86.02%, 8,129/9,450 points)
• verify contracts/v11 and contracts/strategy/v9 are unchanged
• verify deterministic v3-v12 journals and batch/stream conformance fixtures