perf: warm-on-write read caching + leaf-split to fix reindex-churn read regression

[bplatz]

Problem

Under continuous writes with a low reindex_min_bytes, frequent reindexing was
hurting read latency instead of helping — the opposite of the expected
"binary index ≫ novelty query" behavior. A sweep showed the default (100 KB)
churn regime collapsing product-detail read latency ~30× versus a 2 MB setting.

Root cause was not cache invalidation (the shared cache is a single global Arc
and CID-keyed, so unchanged warmth survives a swap). It was that every reindex
swap left the touched artifacts cold — the incremental build decoded the leaves
it rewrote but never seeded the shared read cache — compounded by local-read I/O
amplification and unbounded trailing-leaf growth.

Changes

• Warm-on-write: the incremental CoW build now seeds the shared LeafletCache
  with the leaves it just wrote (decoded under ColumnSet::ALL) and warms the
  reverse-dict leaves, so the first read after a swap hits warm instead of
  cold-decoding. Wired through a late-bound WarmCacheSource at a single
  chokepoint (start_background_indexing_dyn) so every build path is covered.
• Superset cache fallback: a narrow (projection-keyed) read is served from a
  cached ALL batch via ColumnBatch::project_to (invariant: cached ⊇ requested),
  so warming does not duplicate column data across cache entries.
• Bounded incremental leaves: oversized touched leaves are split during the
  incremental build — gap-free, with first_key → next-first-key boundaries and
  ±∞ sentinels at the ends — fixing unbounded growth of the trailing leaf.
• Local read path: mmap local leaf reads and cache the decoded directory.
  Previously a local read did std::fs::read on the whole leaf to serve one of
  its leaflets and re-decoded the directory, bypassing the CID-keyed LeafDir
  cache.
• Within-leaflet seek: binary-search the bound leading key within a leaflet
  scan instead of scanning the leaflet linearly to find the subject's rows.

Bench

Adds reindex_swap_read_profile, a deterministic read-after-reindex-swap harness
that isolates the per-swap cold cost as a single number (not inferred from QMpH),
with first-touch and cold-subject controls.

Validation

Existing-data reads after a churn swap drop from ~3 ms to ~0.2–0.3 ms (~10×) in
the read-after-swap microbench, with cold-miss cache insertions driven toward
zero. Query correctness is unchanged (query and api group suites pass).

Scope note

A follow-on experiment — warming the per-generation query caches (overlay
translation + stats view) at index apply — was prototyped and dropped: it is flat
on BSBM update (the between-swap overlay is small and the query:swap ratio is
high, so the per-swap translation cost amortizes to noise), and the remaining
default-regime cost is the co-located background build competing for CPU, which is
a separate lever tracked for later.