rel_path is backslash-separated on Windows: to_rel_path uses os.path.relpath and nothing normalises to POSIX form

`rel_path` is part of the API contract — `test_rel_path_api_parity.py` exists to hold it — and on Windows it is built from `os.path.relpath`, which returns backslash-separated paths.

**The chain.** `_enrich_with_rel_path()` (`palinode/api/search_helpers.py`) is what attaches `rel_path` to search, associative, dedup-suggest, orphan-repair and topic-coverage results. It calls `to_rel_path()` (`palinode/core/path_guard.py`), whose body is `os.path.relpath(file_path, base)`. That is correct POSIX behaviour and correct Windows behaviour — it is just not the same string, and **`as_posix()` appears nowhere in the package.**

**Seven `test_rel_path_api_parity` tests fail on native Windows** in @Kaap10's 3.12 inventory and @kevin-lozada-santos's 3.11 inventory, both posted on #169. Whether all seven trace to this expression is for the fix to establish — **that has not been verified, and it should not be assumed.**

**A separate, smaller instance in the same family.** `palinode/core/store.py` derives a path for associative results with `fp.replace(config.memory_dir + "/", "").lstrip("/")` — a hardcoded separator. On Windows that replace never matches, so the value stays absolute. It feeds the result's `category` field, **not** `rel_path`, so the consequence is a wrong category rather than a broken contract. Worth fixing in the same pass; worth not conflating with the above.

**The precedent is already in the tree.** `palinode/core/skip_dirs.py` does `rel_path.replace(os.sep, "/")`. Someone solved this once, in one place, and it was never generalised.

**What is wanted:** `rel_path` renders in POSIX form on every platform, and a test that pins it so the next refactor cannot quietly undo it. Where the normalisation belongs — inside `to_rel_path`, or at the enrichment boundary — is a design call worth stating in the PR.

**Not in scope:** the rest of the Windows suite failures. Those are being carved separately.

**Context:** found by @Kaap10 and @kevin-lozada-santos during the native Windows runs on #169.

**Reserved for @Kaap10**, who claimed this slice on #169. GitHub currently rejects the direct assignment on this new issue; it is not open for another claim.

Start with palinode/core/path_guard.py and palinode/api/search_helpers.py, then run test_rel_path_api_parity.py on Windows to confirm which cases fail. Review the separator precedent in palinode/core/skip_dirs.py and the associative path logic in palinode/core/store.py. Done means rel_path is POSIX-formatted across platforms, the contract is pinned by tests, and the separate category issue is handled or explicitly excluded.

Written by the indexing model from the issue text.