fix(srt): tolerate concurrent workspace removals

[ZhiXiao-Lin]

Summary

• treat NotFound during SRT workspace policy scans as a concurrently removed entry
• keep permission and all other I/O failures fatal with their context chain
• add deterministic error-classification coverage and a real SRT concurrent-churn probe
• document the repaired startup behavior

Validation

• cargo fmt --all
• cargo test -p a3s-code-core
• cargo clippy -p a3s-code-core --all-targets -- -D warnings
• real Homebrew SRT 0.0.66 churn probe: 50/50 with Node 24.14.0
• real Homebrew SRT 0.0.66 churn probe: 50/50 with Node 26.5.0
• A/B control on unpatched origin/main: reproduced No such file or directory (os error 2) while opening an already-enqueued directory