fix: remove the glyph-spacing seed from the browser and the launcher

anti-font-fingerprinting.patch added a seeded amount to every glyph advance,
so that text widths differed per context. No real machine produces those
widths: the same font on the same OS measures the same everywhere. So the
noise was itself a fingerprint, measured in #779 at +1 px per ~100 glyphs
plus fractional deltas on every measureText. #779 defaulted the seed to 0
and kept it as an opt-in, but an opt-in whose only effect is to become
detectable is not worth carrying. Removed:

- The browser side:
  - FontSpacingSeedManager and window.setFontSpacingSeed;
  - the HarfBuzz hook;
  - the plumbing that existed only to carry the context id down to the
    shaper: the userContextId on gfxTextRun, gfxShapedWord and the word-cache
    key, and the extra MakeTextRun argument in nsTextFrame, nsFontMetrics,
    MathML and canvas.
  The font group keeps its userContextId, which font-list-spoofing.patch
  uses to apply the per-context font list. Text is now shaped exactly as
  stock Firefox shapes it.
- The fonts:spacing_seed key. The launcher had been sending 0 on every
  launch, plus a setFontSpacingSeed(0) call in every context's init script.
- tests/patches/config-overrides.py, which tested only the spacing override.
  A pythonlib test now covers config_overrides with another key.

timezone-spoofing, webrtc-ip-spoofing and window-setter-seal change only in
context lines and the setter seal list. Every patch applies cleanly to a
fresh tree, and the result builds. The settled decision is recorded as
no-glyph-spacing-noise, with an automated check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pythonlib/tests/test_identity_salt.py

@@ -178,3 +178,19 @@ def test_fingerprint_preset_off_never_draws_a_preset(off):
     checked with `is not None`, so False drew a random bundled preset."""
     with mock.patch.object(utils, "get_random_preset", side_effect=AssertionError("preset drawn")):
         launch(fingerprint_preset=off)
+
+
+def test_no_glyph_spacing_seed_is_generated():
+    """Glyph-spacing noise moved every measured text width off what the same
+    font gives on a real machine, so it was itself a fingerprint; the feature
+    is gone from the browser, and the launcher sends nothing for it."""
+    assert "fonts:spacing_seed" not in launch()
+    context = fp.generate_context_fingerprint(os="linux")
+    assert "fonts:spacing_seed" not in context["config"]
+    assert "setFontSpacingSeed" not in context["init_script"]
+
+
+def test_config_overrides_reach_the_config_and_the_init_script():
+    context = fp.generate_context_fingerprint(os="linux", config_overrides={"audio:seed": 7})
+    assert context["config"]["audio:seed"] == 7
+    assert "setAudioFingerprintSeed(7)" in context["init_script"]