build(dev): generate a 50k-track library and measure a running app
Plan 007 phase 4 is verified by measurement, not by assertion, and there was no way to produce a number: the fixture library is a few dozen tracks and cannot show any of the findings. - `cmd/gentestdata -bulk N` (`make bulkdata`) writes a ~50 000-track library in 11 s / 466 MB by encoding six clips once and copying them, while still tagging every file through `backend/tagwriter` — a library the app cannot read back measures nothing. - `make sandbox-seed-bulk` seeds from it through the same script and the same discipline as any other seed: by running the app and waiting for the real scan. - `e2e/perf/measure.mjs` (`make perf LABEL=x`, `make perf-compare`) takes fourteen measurements against a running app and writes them to a gitignored `.dev/perf/<label>.json`. It wraps every bound Go method, so "did that refetch the library" is a fact rather than an inference, and records `longtask` entries, which is where a 25 MB JSON parse on the main thread shows up and nowhere else. It is not a spec and does not run in CI.
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