perf: content-hash based cache keys for CI compatibility

[zzzz465]

Replace mtime-based cache invalidation with content hashing:

• artifact_cache: use SHA-256 of file content instead of ModTime
• discovery_cache: use content hash for file matching, add WIRE_DISCOVERY_CACHE_DIR env var
• Bump cache versions (artifact v4, discovery v4)

This enables wire cache to work correctly in CI environments where file mtimes are not preserved across runs (e.g., S3 cache restore, git checkout).

[cmilesdev]

Hey! Thanks for contributing.

Main concern for this would be overhead introduced from doing content based hashing. Ran the benchmarks locally and didn't seem to make a substantial difference to cause any concern.

There's a few cache knobs that I think we should unify base env vars on but I'm totally fine with that change as well.

I'm glad the new loader changes has made quite a bit of difference for you. I imagine this was on some pubg services?

scripts/import-benchmarks.sh scenarios

+---------------+-------+--------+----------+----------------------+----------+----------+---------+
|       profile | local | stdlib | external | change type          | stock    | current  | speedup |
+---------------+-------+--------+----------+----------------------+----------+----------+---------+
|         local | 41    | 191    | 1        | cold run             | 379.0ms  | 3088.8ms | 0.12x   |
|         local | 41    | 191    | 1        | unchanged rerun      | 341.8ms  | 9.1ms    | 37.65x  |
|         local | 41    | 191    | 1        | body-only local edit | 325.5ms  | 36.8ms   | 8.85x   |
|         local | 41    | 191    | 1        | shape change         | 324.8ms  | 149.3ms  | 2.18x   |
|         local | 41    | 191    | 1        | import change        | 359.7ms  | 149.0ms  | 2.41x   |
|         local | 41    | 191    | 1        | known import toggle  | 326.4ms  | 149.6ms  | 2.18x   |
|    local-high | 1016  | 191    | 1        | cold run             | 741.9ms  | 5134.3ms | 0.14x   |
|    local-high | 1016  | 191    | 1        | unchanged rerun      | 603.9ms  | 81.6ms   | 7.40x   |
|    local-high | 1016  | 191    | 1        | body-only local edit | 599.5ms  | 173.5ms  | 3.45x   |
|    local-high | 1016  | 191    | 1        | shape change         | 587.3ms  | 690.3ms  | 0.85x   |
|    local-high | 1016  | 191    | 1        | import change        | 591.0ms  | 696.3ms  | 0.85x   |
|    local-high | 1016  | 191    | 1        | known import toggle  | 610.3ms  | 677.8ms  | 0.90x   |
|  external-low | 42    | 243    | 342      | cold run             | 1588.3ms | 8043.6ms | 0.20x   |
|  external-low | 42    | 243    | 342      | unchanged rerun      | 1216.7ms | 17.6ms   | 69.05x  |
|  external-low | 42    | 243    | 342      | body-only local edit | 1172.3ms | 115.0ms  | 10.19x  |
|  external-low | 42    | 243    | 342      | shape change         | 1233.5ms | 421.0ms  | 2.93x   |
|  external-low | 42    | 243    | 342      | import change        | 1253.9ms | 426.1ms  | 2.94x   |
|  external-low | 42    | 243    | 342      | known import toggle  | 1200.1ms | 412.3ms  | 2.91x   |
| external-high | 117   | 243    | 342      | cold run             | 1565.6ms | 8419.3ms | 0.19x   |
| external-high | 117   | 243    | 342      | unchanged rerun      | 1302.9ms | 22.6ms   | 57.74x  |
| external-high | 117   | 243    | 342      | body-only local edit | 1173.0ms | 125.5ms  | 9.34x   |
| external-high | 117   | 243    | 342      | shape change         | 1247.7ms | 460.6ms  | 2.71x   |
| external-high | 117   | 243    | 342      | import change        | 1100.0ms | 464.9ms  | 2.37x   |
| external-high | 117   | 243    | 342      | known import toggle  | 1172.0ms | 465.0ms  | 2.52x   |
+---------------+-------+--------+----------+----------------------+----------+----------+---------+