Add config_overrides to generate_context_fingerprint(), fix font spacing disable (#601)

* Add system-ui font spoofing patch

GetSystemUIFontFamilies() now reads navigator.platform from MaskConfig
and returns the appropriate system font (Helvetica for macOS, Segoe UI
for Windows) before falling through to LookAndFeel::GetFont. Fixes the
CreepJS headless.systemFonts "Sans:Linux" leak when spoofing macOS from
Linux.

* Add patch verification test for system-ui font spoofing

Tests macOS and Windows presets: launches Camoufox with a fingerprint,
measures canvas text width with unquoted system-ui, asserts it matches
the expected system font (Helvetica / Segoe UI).

* Rename test to match patch naming convention

tests/patches/ files should mirror patch names, not use dated prefixes.

* Add config_overrides param to generate_context_fingerprint()

There was no clean way to override config values (like disabling font
spacing perturbation with fonts:spacing_seed=0) because init_script is
rendered inside generate_context_fingerprint() — by the time the caller
gets the config dict back, the init_script string is already baked.

config_overrides is applied after all config building (preset/seeds/
timezone/locale) but before init_script rendering, giving callers a
clean override point without touching the preset (which represents
real device data, not perturbation config).

The longer-term fix is separating config building from init_script
rendering so callers can modify config and re-render. config_overrides
is the minimal API addition that unblocks the use case without that
refactor.

* Rename test to match patch naming convention

pythonlib/camoufox/fingerprints.py

@@ -423,6 +423,7 @@ def generate_context_fingerprint(
     webrtc_ip: Optional[str] = None,
     timezone: Optional[str] = None,
     locale: Optional[str] = None,
+    config_overrides: Optional[Dict[str, Any]] = None,
 ) -> Dict[str, Any]:
     """
     Generate fingerprint values for a single per-context identity.
@@ -438,6 +439,9 @@ def generate_context_fingerprint(
         locale: BCP-47 locale string (e.g. 'en-GB'). When provided, parsed via
             normalize_locale() and injected into config. Also sets
             context_options['locale'] for Playwright.
+        config_overrides: Dict of CAMOU_CONFIG keys to override after config
+            is built but before init_script is rendered. Useful for disabling
+            perturbation (e.g. {'fonts:spacing_seed': 0}).
     """
     if preset is not None:
         # Use real fingerprint preset
@@ -535,6 +539,10 @@ def generate_context_fingerprint(
         if parsed.script:
             config['locale:script'] = parsed.script
 
+    # Apply caller overrides before rendering init_script
+    if config_overrides:
+        config.update(config_overrides)
+
     # Build the values dict for the init script (works for both paths)
     init_values: Dict[str, Any] = {
         'fontSpacingSeed': config.get('fonts:spacing_seed'),