fix(vendor): recover Windows portability patches

[Nanako0129]

Summary

• Recover the eleven tokscale-core commits validated in the Windows downstream so TokenBar remains the canonical source for the next Windows sync.
• Release the temporary cache writer before Windows atomic replacement and reopen the final cache read/write before the durability sync.
• Keep Windows Zed and config fallbacks behind use_env_roots, and evaluate extra-path warnings against the supplied scan home.
• Isolate core fixtures from inherited environment variables, client roots, current directories, cache directories, and user-profile parser variants.
• Record the downstream provenance and upstream follow-up split in vendor/README.md.

Provenance

The Windows port started from TokenBar commit 2ed256ee. This PR recovers the validated vendor sequence documented in the issue #45 handoff:

e5200634 -> db2a96a3 -> 15f418ee -> e807f333 -> 0979cdb0 -> 26b892a6 -> 6ac77c03 -> e91fb2c6 -> fbecb99c -> 3c8bfc52 -> aec5bd88

The seven modified vendor source files are byte-identical to the Windows tree at aec5bd88. The ledger is the only additional file; this is a selective recovery, not a wholesale re-vendor.

Compatibility

• CACHE_SCHEMA_VERSION remains 29.
• No serialized parser output, cache layout, incremental state, FFI, C ABI, Swift, dependency, or public API contract changes.
• The change has no path overlap with open PR feat(quota): generalize historical pace across providers #58.
• Upstream work remains split into cache runtime, explicit-home scanner, and test-only hermetic follow-ups; this PR does not report any of them as upstreamed.

Verification

• Nine focused Rust regressions covering panic-safe guards, cache persistence, explicit-home isolation, positive environment behavior, Claude variants, and materialized parser fixtures.
• Hostile-environment default-parallel core suite: 1,058 passed, 1 ignored; Codebuff 10 passed; Hermes 3 passed; streaming snapshot 1 passed.
• Real profile metadata and content-digest manifests remained unchanged.
• cargo test --workspace
• cargo clippy --workspace --all-targets
• make build
• swift run TokenBar --selftest
• swift run TokenBar --smoke (exit 0; existing live-data drift and unavailable-provider diagnostics were still emitted)
• make check-docs
• git diff --check
• Fresh-context outcome verification: CONFIRMED

cargo fmt --all -- --check is not green: untouched origin/main already has formatter drift, and preserving the Windows-validated byte-identical files retains additional rustfmt suggestions. This PR intentionally does not reformat those vendored files; every other listed gate passed.