fix(desktop): atomically publish staged providers

[BrianInAz]

Summary

• copy provider bytes into an unpublished candidate path
• sync, protect, and close the candidate before atomically publishing the executable pathname
• preserve immutable-byte and Windows sharing contracts while eliminating Linux ETXTBSY execution races

TDD evidence

• Red: the new publication contract failed to compile until candidate-copy and atomic-publish helpers existed; PR fix(ci): bootstrap Sprig rolling release #29 job 93664535019 supplied the runtime ETXTBSY Red
• Green: candidate publication contract and all 8 provider-focused tests pass
• Green: exact former failing deployment test passes 20 consecutive runs
• Green: cargo fmt and clippy with warnings denied
• Green: full Desktop Tauri suite (2,116 passed, 15 expected ignored; 3 diagnostics passed)
• Green: full just ci including 1,098 mobile tests with 1 expected skip

Tracking

• Beads: ios-buzz-59e.13
• Unblocks PR fix(ci): bootstrap Sprig rolling release #29 after merge and post-merge main validation

No sleeps, retries, ignored tests, platform skips, workflow narrowing, or Sprig changes were introduced.

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