fix(storage): preserve concurrent Windows portable destinations

[DecisionNerd]

Windows portable publication used std::fs::rename, which can replace a destination created after export staging begins. Route publication through a Rust-owned atomic no-replace operation using MoveFileExW with zero flags; preserve the existing Unix primitive and typed export failures.

Closes #1190.

File and directory sentinel tests verify that existing bytes survive refusal. A synchronized two-writer test proves exactly one publisher wins. The Windows lane also runs the export race regression; the mapped expanded/bundle export, verify, import, and reopen coverage inherited from #1188 checks successful publication.

Validation on the refreshed branch:

• cargo test -p graphforge-filesystem --lib: 33 passed.
• cargo test -p graphforge-storage --lib project_portable_v2_export::tests::: 25 passed, including destination replacement and successful expanded/bundle round trips. Durable fixtures used TMPDIR on the host root ext4 filesystem.
• cargo clippy -p graphforge-filesystem --all-targets -- -D warnings: passed.
• cargo fmt --all -- --check, make pre-push-fast, and make gate-registry-check: passed.
• Independent review found no blocking issue; rebase range-diff confirms the patch is unchanged atop merged fix(storage): encode semantic routes into portable components #1188.

Exact-head hosted CI, including Windows and macOS, remains the merge gate. Full local make pre-push is not claimed green; the existing host tmpfs fixture limitation is tracked by #1192.

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