Fix SLR download failures and CMake rebuild detection

Replace SHA256SUMS verification with Content-Length check for SLR
downloads. Valve's CDN frequently serves stale SHA256SUMS that don't
match the current archive, causing persistent checksum mismatches.

Fix CMake DEPENDS for nak_ffi to glob all .rs sources instead of
hardcoding a few files — previously changes to slr.rs (and most other
Rust files) were silently ignored by ninja.

Also wrap prefix init and runtime commands through SLR pressure-vessel
container, and expose Proton install directories to the container.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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