perf: round 15 — grouped-listing O(N²) rebucket, exif/reseed allocs, tantivy zero-hit snippet skip

Benchmark-gated, same rule as rounds 2–14: every change ships with a
BEFORE/AFTER benchmark and an equivalence/safety gate; an AFTER that doesn't
beat its BEFORE is rolled back. The rule is encoded per harness (GATE FAIL
non-zero exit in the Rust examples, threshold expect() in vitest).

F1 — Grouped listings (trash / recent / favorites / shared-with-me)
re-bucketed the WHOLE accumulated list on every infinite-scroll page.
ResourceSectionsBuilder (new, off the reactive graph) re-buckets only the
fresh page and hands VirtualList the same rows array reference for untouched
buckets. 50×50 (2 500-item) drain: 63 750 → 2 500 bucketOf calls (25.5×),
12.5 → 1.3 ms wall (9.9×); O(N²/page) → O(N). Deep-equal to the full-rebuild
reference at every page for both a contiguous (date) and a non-contiguous
(trash-by-drive) group-by; reference-stability + fallback gated.

B1 — exif Make/Model: the display String was thrown away to allocate the
trimmed copy; display_value_trimmed trims in place (drain + truncate), 2 → 1
alloc per field (8 → 4 allocs/op, 1.26×).

B2 — content-index worker: text_extractor::supports (lowercases MIME +
extension) was called twice per file per drain batch; classify once into a
Vec<bool> and thread it through both uses. 256-file batch: 704 → 353 allocs,
34.5 → 16.7 µs (2.07×).

B3 — tantivy: skip SnippetGenerator::create on a zero-hit content search
(return Ok(vec![]) once top_docs.is_empty()); the per-hit loop was empty.
400-doc index: 1 575.6 → 1 237.2 ns (1.27×), widens with index size.

Harnesses: examples/bench_round15_micro.rs, examples/bench_round15_tantivy.rs,
frontend resourceSections.bench.test.ts; writeup in benches/ROUND15.md. Also
normalizes two round14 bench examples that were committed unformatted
(cargo fmt --all).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012o47jSrtL7xuNGTHXmtiYL

Cargo.toml

@@ -354,6 +354,24 @@ name = "bench_micro_allocs"
 path = "examples/bench_micro_allocs.rs"
 required-features = ["bench"]
 
+# Round-15 battery ────────────────────────────────────────────────────────────
+
+# Round-15 CPU/alloc micro-pack — exif Make/Model in-place trim (drop the
+# throwaway display String), content-index worker single supports() classify
+# per file (was called twice per drain batch). No Postgres.
+[[example]]
+name = "bench_round15_micro"
+path = "examples/bench_round15_micro.rs"
+required-features = ["bench"]
+
+# Round-15 tantivy zero-hit snippet skip — the SnippetGenerator::create the
+# search path builds even when the query matched no documents (pure waste on a
+# no-hit content search). Builds a RAM index; no Postgres.
+[[example]]
+name = "bench_round15_tantivy"
+path = "examples/bench_round15_tantivy.rs"
+required-features = ["bench"]
+
 # Round-14 battery ────────────────────────────────────────────────────────────
 
 # Round-14 query-shape pack — lightbox face-box narrow projection (drop the