feat(mem-wal): snapshot-consistent as-of cut for fresh-tier membership

[hamersaw]

What

Adds AsOfCut { active_generation, active_batch_count } and LsmScanner::contains_pks_as_of (with fresh_tier_block_list threading the cut) so a caller can evaluate fresh-tier PK membership against the exact tier a prior scan observed, instead of the live tier.

Why

Sophon's WAL block-list runs as two independent RPCs (the read arm and a supersession check) that each snapshot the live fresh tier at their own call time. Under concurrent writes the two snapshots disagree, so a base row can be dropped as "superseded" by the check while the arm never delivered a replacement — a transient missing row. The fix pins both phases to the same cut; this PR is the lance half that lets the check reconstruct the arm's snapshot.

How the cut works

The active memtable is the only fresh-tier source that grows between two reads; everything strictly below its generation (frozen memtables, flushed generations) is immutable. So the as-of filter:

• includes in-memory/flushed sources below active_generation whole (immutable, fully observed),
• bounds the active generation to its first active_batch_count batches (by append index),
• excludes in-memory sources above active_generation and flushed generations >= active_generation (produced after the snapshot).

It uses only batch_store.len() and the memtable generation — both always available on the read path — and only ever excludes rows the scan did not observe, so a stale cut under-counts (a tolerable stale read) rather than over-counts (which would drop a row with no replacement).

Note: an earlier approach keyed the cut on per-batch WAL positions (wal_batch_mapping), but that map is only populated by mark_wal_flushed, which is test/bench-only — empty in production. The generation + batch-count cut avoids any write-path dependency.

Tests

fresh_tier_block_list as-of unit tests: active-memtable batch-count bound, newer-gen-excluded / lower-gen-included-whole, flushed-gen at/above active excluded.

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