fix(rust): slash-separate root-relative paths on Windows

[thekevinbot]

Fixes #1184

Root-relative paths are built with scanner::to_slash, so they are /-separated on every platform. Applied where they are produced: the initial scan (relative_path), the watcher (process_file_event, index_subtree), the path-table vtab (reported_path) and the parsed-vtab cache key and skip messages. Db::files_under now matches on a / prefix, since stored paths no longer use the native separator.

reported_path joins a path prefix with / and leaves the prefix as given, so an absolute path-table reads <prefix>/docs/a.md, as path_table_globs.rs expects.

tests/watch_scan_correctness.rs fixture change, justified: its on-file closure computed path by stripping the canonicalized root and keeping native separators. On Windows the scan hands it paths under the root as given (C:\Users\RUNNER~1\...) while canonicalize returns \\?\C:\Users\runneradmin\..., so the strip failed and rows held full paths. The fixture now strips either spelling and slash-separates, which is what it meant.

Glob matching needed no change: globset already normalizes \ to / on Windows before matching.

to_slash delegates to with_slashes(path, native_separator). The separator is a parameter so a Linux unit test covers the Windows case. On Unix it is a no-op, because \ is a legal file-name character there.

Windows failures

Before (run 35995035941): 99 failing. After (run 36436772830): 51 failing, none of them #1184. The comment below lists the issue that owns each one.

E2E Verification

• Ran e2e suites locally for every affected SDK
• Python SDK e2e: pass (38 passed)
• TypeScript SDK e2e: pass
• Rust core e2e: pass (cargo test -p dirsql --features cli, 0 failures)
• packages/python/e2e-attestations/claude-1184-win-rel-paths.json receipt written (just e2e-attest-python)
• packages/ts/e2e-attestations/claude-1184-win-rel-paths.json receipt written (just e2e-attest-ts)
• Command(s) run: just e2e-attest-python, just e2e-attest-ts, cargo test -p dirsql --features cli, just preflight
• Result summary: all green on Linux. Manual run: dirsql "SELECT path, dir FROM '/tmp/claude/win1184-demo/docs'" returned /tmp/claude/win1184-demo/docs/nested/a.md and dir /tmp/claude/win1184-demo/docs/nested, unchanged on Linux.

Changelog / Migrations

• Changelog fragment added under packages/rust/changelog.d/
• Migration fragment added under packages/rust/migrations.d/ (Windows-only behavior change; Unix output is unchanged)

[thekevinbot]

Windows tests: 99 failing on run 35995035941, 51 failing on this PR (run 36436772830). All the failures #1184 names now pass: db::tests::files_under_*, internal_tests::process_file_event_*, vtab::tests::*, vtab.rs, path_table_query.rs, path_table_gitignore.rs, most of path_table_globs.rs, watch_scan_correctness.rs, and several cli_e2e.rs tests.

The 51 still failing, by owning issue:

#1185, absolute / ~/ / ../ path-tables (11): path_table::tests::{a_home_relative_*, a_parent_relative_path_folds_*, a_parent_relative_path_past_*, an_absolute_glob_*, an_absolute_single_file_*}, path_table_cli_e2e::an_absolute_path_table_resolves_outside_the_index_root, and path_table_globs::{a_parent_relative_*, an_absolute_*} (5). a_parent_relative_path_table_resolves_against_the_index_root now differs only in the prefix, which normalize rewrote to \: it produces ...\.tmpX\docs/a.md where the test expects ...\.tmpX/docs/a.md.

#1186, test harness (38):

• On-file fixtures produce invalid JSON: they put a native {path} into the JSON or strip "$2"/ from it. This affects internal_tests::{build_tables_from_config_*, run_on_file_*} (4), from_config.rs (7), config_root_derivation.rs (2), declared_table_name.rs (2), and cli_e2e::{get_events_emits_*, query_subcommand_fans_out_*, server_indexes_a_directory_moved_*}, plus cli_integration::get_events_streams_mutation_events.
• on-file = "printf '[{}]'" indexes 0 rows with no error. This hits sdk::builder_* (3), async_sdk::from_config_path_*, cli_repeatable_config_e2e.rs (2), cli_e2e::config_elsewhere_*, and repeatable_config.rs (2). I have not confirmed the cause. The likeliest one is MSYS argv globbing brace-expanding [{}] to []. Windows: Rust test fixtures assume Unix paths and file handles #1186 does not list this yet.
• persist_parsed_path_table.rs (8): two tests hit set_modified PermissionDenied, which Windows: Rust test fixtures assume Unix paths and file handles #1186 lists. The other six interpolate a native script path into the command template, and shlex eats the backslashes (C:UsersRUNNER~1...parse.sh). Windows: Rust test fixtures assume Unix paths and file handles #1186 does not list that yet.
• extensions.rs TOML escape (1). parsed_vtab::run_parser_* cwd os error 267 (1).

Unowned, Unix absolute paths in unit fixtures (2): internal_tests::relative_root_canonicalizes_watch_root_only (/ws/canonical is not absolute on Windows) and extension_resolution::tests::select_loadable_rejects_an_ambiguous_package_in_sorted_order (/pkg joined natively as /pkg\a.so). Neither is root-relative. Both fit #1186 best.