Purpose
-------
OI-533 regression tests — :func:`oct.core.paths.to_project_relative`
renders filesystem paths as project-relative POSIX strings for
user-facing output (lint findings, health reports, git status, etc.)
so absolute machine paths never leak into CI logs or pasted errors.
Responsibilities
----------------
- In-tree paths are rendered relative to *project_root*.
- Out-of-tree paths fall back to the absolute POSIX form.
- Windows-style backslashes are normalised to forward slashes on
every platform.
Diagnostics
-----------
Domain: CORE-TESTS
Levels:
L2 — test lifecycle
L3 — individual assertions
L4 — deep tracing
Contracts
---------
- Tests use synthetic paths; no real filesystem reads or writes.