shard-serialization P3.13: add instance-local suppression state for SkipBoth

[romange]

Phase 3 — Bring non-PIT onto the new foundation (Task 13 of 17)

Goal: Keep the optimization entirely within the existing per-instance callback pair, without cross-instance coordination.

Design doc: docs/shard-serialization.md — Technical Roadmap

Dependencies

• shard-serialization P1.3: introduce bucket-level completion state machine #6830 (P1.3: bucket-level completion state)
• shard-serialization P1.5: associate delayed tiered entries with bucket state #6832 (P1.5: delayed tiered entries with bucket state)
• shard-serialization P3.11: stamp bucket version in non-PIT mode #6838 (P3.11: non-PIT version stamping)

Description

Add per-snapshot-instance state that lets `OnDbChange` record a suppression decision (SkipBoth) for mutations whose effects will be covered by future traversal, and lets the same instance's `ConsumeJournalChange` consult and clear that state.

Guidelines

Architecture

• Each `SliceSnapshot` (or `RestoreStreamer`) instance has its own `OnDbChange` and `ConsumeJournalChange`.
• The suppression decision is instance-local: multiple snapshots on the same shard make independent decisions based on their own `snapshot_version_` and traversal progress.
• No shard-wide or cross-instance aggregation.

Suppression state

• Add a set/map to `SliceSnapshot` that records suppressed mutations (keyed by LSN or key name).
• `OnDbChange` adds entries when SkipBoth conditions are met (conditions defined in task 14).
• `ConsumeJournalChange` checks the set: if the mutation is suppressed, skip the journal write and clear the entry.

Scope

• This task adds the mechanism only — the suppression set and the check/clear logic.
• The actual eligibility logic (when to set SkipBoth) comes in task 14.
• Initially the set remains empty (no mutations are suppressed yet).

Acceptance criteria

• Suppression state container added to `SliceSnapshot`.
• `ConsumeJournalChange` checks suppression state and skips suppressed entries.
• No mutations are actually suppressed yet (set is always empty).
• All existing tests pass unchanged.
• No cross-instance or shard-wide state introduced.