perf: ctx score --against HEAD is 3x over its hook-path budget on the 2k-file fixture

[saldestechnology]

Measured

score_3_changed (score with 3 changed files on the 2k-file fixture), ubuntu-latest CI, perf profile:

Run	Suite	Median	Max RSS
29124622840	pr	6576.5 ms	273.1 MB
29126075011	full	5965.1 ms	273.8 MB

Budget is 2000 ms / 300 MB. With the CI budget scale of 1.5, the allowed median is 3000 ms — the measured medians are roughly 2x the scaled allowance, 3x the raw budget. Locally on macOS it is about 4.2 s / 379 MB, so this is not a CI-runner artifact.

Why it matters

ctx score --against HEAD is on the Stop-hook path: under the harness it runs on every session stop, so its latency is paid at the end of every agent session. Every other hook-path scenario is comfortably within budget on the same runs:

• check_against_head: 163 ms (budget 1000 ms)
• map_cached: 46 ms (budget 500 ms)
• index_incremental_1: 210 ms (budget 300 ms)
• sql_v1_query: 266 ms (budget 500 ms)

score is the outlier by more than an order of magnitude.

Likely suspects

• score recomputes near-duplicate fingerprints over the whole index on every invocation.
• Baseline-side symbol metrics are recomputed per file rather than reused.
• No caching between hook invocations, even though the index and baseline rarely change between consecutive stops.

Consequence

The perf CI job stays advisory (continue-on-error: true) until this is fixed. Flipping the job to required is blocked on this issue.