Confine cache paths and harden the v0.1.2 release

[CAOShurong]

What changed

• reject project names outside the Python packaging name grammar before any network or cache work
• resolve every cache destination and prove it remains under the selected cache before deletion or creation
• add regression coverage for project-name traversal, version traversal, valid names, and Unicode case-folding lookalikes
• test Python 3.14, pin all third-party Actions, add Actions Dependabot, and attach checksums plus build provenance to releases

Why

A crafted package or version could previously make the cache path resolve outside the selected cache. If a matching directory existed there, the incomplete-cache cleanup could delete it before the network request failed.

Verified

• 96 offline tests
• Ruff lint and format checks
• generated README check
• wheel and sdist build plus strict Twine validation
• fresh-wheel installation reports willitbreak 0.1.2
• real compatible upgrade exits 0
• real urllib3 breaking upgrade exits 2 and names both call sites
• malicious name and version paths exit 1 while outside sentinel files remain intact
• every pinned Action SHA matches its published tag