ci: cache generate-lockfiles to skip ~47s on unchanged deps (#1076)

[lmeyerov]

Summary

• Adds `actions/cache@v4` to the `generate-lockfiles` job, keyed on `setup.py`, `pyproject.toml`, `requirements/*.txt`, and `bin/generate-lockfiles.sh`
• On cache hit, the `uv` install and lockfile generation steps are skipped entirely
• `upload-artifact` always runs so downstream jobs always receive the lockfiles (from cache or freshly generated)
• Adds `cooldown_days` `workflow_dispatch` input (default `6`) — pass `0` to force-refresh all lockfiles; value is included in the cache key so a `cd0` run always misses and stores its own entry

Observed speedup

Run type	`generate-lockfiles` duration
Cold miss (first run / deps changed)	~26s
Cache hit (same deps)	~6s

~20s saved on cache hit (issue estimated ~47s; actual varies by runner load).

Test plan

• Cold miss: CI run 24055215908 — Install uv + Generate lockfiles ran, cache stored
• Cache hit: CI run 24055236377 — Install uv + Generate lockfiles skipped, Upload lockfiles succeeded
• Cache bust (`cooldown_days=0`): CI run 24055315814 — new key `cd0-` missed, generation ran
• All PR checks green

Closes #1076.

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