- env-change-with-unchanged-mtime must re-expand (proves the cache stores raw
un-expanded YAML, not the expanded result -> no stale cross-profile expansion)
- no-cross-serve between two profile config paths (proves the parse cache is
path-keyed). Both pass on the current impl; locks down the highest-risk
cross-profile credential-bleed invariant per Opus's non-blocking note.
The 'and loaded' guard skipped the _cfg_mtime stamp for a config that parses to
an empty {} dict, so get_config()'s current_mtime != _cfg_mtime stale check fired
on every call and re-entered reload_config() under _cfg_lock forever — a hot-path
spin for a freshly created/reset profile's {} config. Split the guard: the no-op
_cfg_cache.update is still skipped for {}, but _cfg_mtime is always stamped when
the file parsed to a dict, matching master's pre-#4662 behavior. Incidentally
fixes the pre-existing empty/None-config spin. Adds a regression test.
Phase 2 of #4662. reload_config() did a raw yaml.safe_load of config.yaml on
every call (~125ms+ on a large file), including on the profile-switch hot path
(~446ms load_settings stage in /api/sessions diagnostics). It now parses through
the mtime+size-keyed cache added in #4652, via a shared _load_yaml_config_file_raw
core (DRY: _load_yaml_config_file = raw + env-expand). reload_config still runs
its OWN env expansion pinned to the unscoped process-env view (block_process_env_fallback
reset + _thread_ctx.env={}) so the #798 TLS profile-isolation invariant is byte-for-byte
preserved; only the disk parse is now cached. An unchanged config.yaml is no longer
reparsed; an mtime change still busts the cache.
