feat(read): route primary-key realtime scans

[HaHaJeff]

Part 8/9 of the primary-key realtime stack.

Depends on: #12

What is this PR for?

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

What changes?

Routes primary-key scans through grouped disk and immutable memory splits while preserving append-only realtime behavior. Adds scan validation and focused resource-lifecycle 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.