feat(backend): enforce Forge process custody

[sandersonstabo]

Summary

• add whole-process Forge custody backed by one retained nonblocking filesystem lock
• reject missing/unsafe parent chains, symlink/reparse paths, and non-regular lock carriers
• report typed same-process and cross-process contention without retrying or unlinking
• register the real backend facade and cross-process Windows test in Cargo/Bazel quality gates

Verification

• registration audit: 257 tests, 276 Clippy members, 267 format roots; zero findings
• locked/offline Cargo metadata and Rust 1.98 rustfmt
• focused //modules/backend:backend + //tests/backend:process_custody_test build
• uncached cross-process custody test passes
• fresh production/test Clippy markers after pedantic corrections
• exhaustive //:format_test
• exact hash/status/diff guards and Bazel shutdown

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Summary by cubic

Adds whole-process Forge custody so two processes can no longer open the same Forge storage concurrently. The backend now acquires one retained nonblocking filesystem lock, rejects missing or unsafe parent chains, symlink/reparse paths, and non-regular lock carriers, and reports typed same- and cross-process contention without retrying or unlinking the lock file.

New Features

• ForgeProcessCustody guard is non-clone and must stay alive through shutdown; missing lock files are created atomically with owner-only permissions.
• Adds a cross-process custody test covering create races, contention, and unsafe path shapes, registered in Cargo and Bazel quality gates.
• Pins fs2 at =0.4.3 for the lock backend.

^{Written for commit 659a1af. Summary will update on new commits.}