feat(read): merge primary-key memory with snapshots

[HaHaJeff]

Part 7/9 of the primary-key realtime stack.

Depends on: #11

What is this PR for?

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

What changes?

Extends merge-file reads to combine primary-key in-memory readers with snapshot files through the existing key-value merge and projection pipeline. Adds focused coverage for ordering, deletes, and resource handling.

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.