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.
