Add reusable uv and Rust cache actions

[calavera]

What changed

• add actions/setup-uv-cache, a composite action that installs uv and Python, restores a compatibility-keyed environment archive, runs uv sync, and publishes cold archives atomically
• add actions/setup-rust-cache, a composite action that configures persistent Cargo, sccache, and optional target directories
• replace the reference workflow cache scripts with one local uses: step
• document concise external usage, inputs, outputs, invalidation, and concurrency guidance for both actions
• constrain setuptools package discovery so non-Python action directories cannot be mistaken for packages

uv cache design

The live virtual environment remains on fast sandbox-local storage. The Tensorlake volume stores one compressed archive keyed by platform, exact Python version, workflow job, Git ref, working directory, and uv.lock.

Running the live environment directly on the volume was rejected after three sequential trials stayed cold and took 2m06s to 2m13s. The archive design avoids Python small-file reads over FUSE and gives volume autosave one sequential object.

Runtime validation

• cold composite-action run: 21s total
• warm composite-action rerun: 13s total, 1.208s restore, 0.857s uv sync
• earlier sequential archive runs: 21s cold, then 12s and 12s warm
• independently remounted archive: 35,890,463 bytes and 3,321 entries

Validation

• PR workflow passes cold and warm through actions/setup-uv-cache
• all action and workflow YAML parsed
• every embedded Bash block passed bash -n
• ruff check and format check passed
• 32 tests passed
• local uv sync rebuilt the Python project successfully with the new top-level action directories
• git diff --check passed