fix: honor fsync for local deletes

[RanaPriyansh]

Fixes #767.

Standalone deletes ignore LocalFileSystem::with_fsync(true).

Pass the configured flag to the deletion helper so direct and streamed deletes sync the parent directory after unlink. Retain the directory handle before unlink to prevent concurrent cleanup from removing the path before sync. Document that automatic directory cleanup is not synced.

Unix regression tests reproduce the missing sync-error propagation on the baseline. Controls cover concurrent deletion with cleanup, disabled fsync, symlink targets, and nested-directory cleanup. Permission tests skip when the host can open a directory without read permission.

Validation: cargo test, cargo fmt --all -- --check, cargo clippy --all-features --all-targets -- -D warnings, cargo check --all-targets --all-features, and the CI Clippy feature matrix passed locally. No power-failure or Windows durability claim is made.

Codex used.