feat(realtime): extend store contracts for primary-key transport

[HaHaJeff]

Part 1/9 of the primary-key realtime stack.

Base commit: 53f9c86

What is this PR for?

Build the primary-key realtime path as one reviewable layer of the stack.

What changes?

Extends the realtime store request and read/write contracts with explicit store mode, memory pool, transport schema, and realtime offset metadata. Tightens context validation and sequence reconciliation, with focused contract and special-field coverage.

Tests

• Restack validation: every commit prefix was built and its focused tests passed.
• Stack-wide validation: the final realtime integration suite passed 58/58, the primary-key integration slice passed 16/16, and the full pre-commit run passed all nine hooks.
• Post-seal validation: the focused WAL replay test was rebuilt and passed after the final formatting amend.