Windows: Rust test fixtures assume Unix paths and file handles #1186

Found by the Windows CI job added in #1178 (run https://github.com/thekevinscott/dirsql/actions/runs/35995035941).

Some Rust test failures on Windows are harness assumptions, not product bugs:

• TOML fixtures interpolate a native path into a basic string, so D:\a\... is an invalid escape (extensions.rs). Use a literal string or forward slashes.
• On-file fixtures echo {path} into a JSON payload unescaped, so a \ path breaks the JSON (internal_tests::run_on_file_*, several cli_e2e.rs tests).
• parsed_vtab::tests::run_parser_substitutes_the_absolute_path_and_the_root spawns with a cwd that does not exist on Windows (os error 267).
• persist_parsed_path_table.rs calls set_modified on a handle opened read-only; Windows needs write access (PermissionDenied).

Fix after the separator and absolute-path issues land, so the remaining reds are only these.