perf(web): stop rebuilding a whole plan for three sample lines

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Review finding #3. The sample endpoint fires whenever the operator selects a
technique in the catalog rail, and it built the entire plan to pick three lines
out of it. REP-004 at high intensity is 180,000 events and about 1.6 seconds of
pure CPU, discarded immediately, on every click.

The reviewer suggested a "build three and stop" path. That would change what the
sample shows: it is deliberately the FIRST, MIDDLE and LAST event, and the middle
of three is not the middle of the run. So the cost is cached rather than the
output truncated.

The cache is exact, not approximate. The request is built from a fixed seed and a
fixed intensity, so (technique, intensity, vendor) determines the lines
completely. Bounded because a server is long-lived, though the whole key space is
24 techniques x 3 intensities x 3 vendors.

Guarded by counting build_plan calls rather than timing, which passes or fails on
how busy the machine is, plus three tests that the cache did not change what a
sample contains or leak across intensity and vendor.

952 py. black, ruff, mypy clean.