ci: gate pull requests on the stress benchmark losing no work

Every ceiling this project has hit only appeared under a mixed load, and
three of the four lost work silently while single-feature e2e stayed
green, yet the benchmark only ever ran by hand on one laptop. With
STRESS_REQUIRE_FUNCTIONAL=1 the stress example exits 1 when a download,
upload, search or browse was lost; throughput never fails it because it
measures the machine, not the client. CI now runs a quarter-size profile
against soulfind on every pull request.

benchmarks/README.md

@@ -32,6 +32,13 @@ with headroom, so the score still moves when transfer handling regresses.
 `--save` only overwrites `stress-baseline.json` when the run beats it, so the
 baseline ratchets upward. Without `--save` a run just prints the delta.
 
+## The gate
+
+`STRESS_REQUIRE_FUNCTIONAL=1` makes the run exit 1 when any of the four
+functional dimensions is below 100%. CI runs it that way on every pull request
+with an eighth of the default load; throughput never fails the gate because it
+measures the machine, not the client.
+
 ## Tuning the load
 
 Every knob is an env var; the defaults are in `Config::from_env`.