Add a benchmark harness (src/project/bench_switcher.cpp, manual target)
that times the Go-to-File/Symbol pipeline against a real project and
writes a metrics file. Measured on mobydick (12.2k files, 17.2k symbols);
findings in docs/PERF.md.
Bottleneck: PaletteModel::setQuery re-scored all N every keystroke
(Debug: 270-600ms/keystroke; cost grew with query length). Fix: when the
query is a pure append, re-score only the currently-visible subset — the
match set is monotonic under appending, so this is exact. Later keystrokes
drop from O(N) to O(matches) (symbols 'server' 30ms -> 9ms in Release).
Guarded by test_palettemodel::incrementalMatchesFullScan.
Also documented: shipped plugin is a Debug build (~5-8x slower than
Release), and ctags/listFiles run ~0.1-1s synchronously on open (move
off-thread = open follow-up).
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