feat(realtime): add primary-key realtime store

[HaHaJeff]

Part 3/9 of the primary-key realtime stack.

Depends on: #7

What is this PR for?

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

What changes?

Adds the default in-memory primary-key realtime store, including batch retention, sealing, commit readers, immutable read views, query projection, committed-offset advancement, and memory accounting. Adds focused store and memory-lifetime 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.