refactor(platform): phase 5/9 migrate filesystem and path mechanics #1311

Parent: #1306
Depends on: phase 4 process migration

Scope

Move host filesystem mechanics behind fbuild_core::platform::fs:

• file identity and same-file behavior;
• link/reparse classification and permissions;
• atomic replacement/open-image retirement;
• volume/mount facts and native error normalization;
• host-specific path mechanics currently embedded in core, package, daemon, and deploy code.

Keep NormalizedPath, cache policy, archive traversal, authorization, and retry policy in their current neutral owners. Delete exact migrated ledger rows.

Acceptance criteria

• RED characterization covers Windows, Linux, and macOS filesystem differences.
• Shared callers contain no native extension traits, raw handles/fds, or concrete OS filesystem modules.
• Existing path normalization, cache identity, install-lock, and atomicity behavior remains compatible.
• Before/after ledger totals and RED -> GREEN evidence are recorded.
• Focused filesystem/path tests, three-host CI, workspace check/clippy, and repo tests are GREEN.