Fix/exec double count followup

[brendanclement]

Addresses Weston's review comments on #6799

• Revert: drop setup-time Instant::now() brackets that captured I/O time. I wrapped setup .awaits in MapIndexExec, FlatMatchFilterExec, and FlatMatchQueryExec with Instant::now() to add_duration(start.elapsed()) to attribute one-shot setup work to elapsed_compute. Weston pointed out those brackets capture I/O wait, not CPU.
• Replace InstrumentedChildInputStream with inline RAII timer in .then() bodies, Deletes the InstrumentedChildInputStream helper struct.
• Extend flat_bm25 scoring timer to pre- and post-await sync work In flat_bm25_search_stream_with_metrics the single scoring_start region only covered initialize_scorer + flat_bm25_score.

Testing

Benchmarked (1M-row synthetic dataset with BTREE + INVERTED + IVF_PQ indexes, identical analyze plan queries on each side). Comparing against the commit prior to this work

Scenario	Node	Before	After	Notes
KNN + BTREE scalar prefilter	KNNVectorDistance (520 rows)	1.39 ms	112 µs	~12× smaller — the original double-count fix preserved
KNN + FTS post-filter	FlatMatchFilter (0 rows)	10.95 ms	42 µs	~262× smaller
KNN + FTS prefilter	FlatMatchQuery (2.52 K rows)	848 µs	79.08 ms	~93× larger; spawn_cpu tokenize CPU now correctly attributed
KNN + FTS prefilter (control)	MatchQuery (untouched code)	3.80 ms	4.63 ms	noise only

[claude]

Claude Code Review

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[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 80.18018% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/exec/fts.rs	64.51%	8 Missing and 3 partials ⚠️
rust/lance/src/io/exec/rowids.rs	83.33%	1 Missing and 4 partials ⚠️
rust/lance-index/src/scalar/inverted/index.rs	75.00%	0 Missing and 2 partials ⚠️
rust/lance/src/io/exec/knn.rs	88.23%	1 Missing and 1 partial ⚠️
rust/lance/src/io/exec/scalar_index.rs	92.00%	1 Missing and 1 partial ⚠️

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