feat: public config helpers for third-party integrations (#25)

Adds invisible_playwright.config module with:
- get_default_stealth_prefs(seed, *, pin, locale, timezone,
  extra_prefs, humanize, virtual_display) -> dict
- get_default_args() -> list

Both also re-exported at the package root alongside the existing
InvisiblePlaywright. ensure_binary is also re-exported there for
parity with the cloakbrowser.download.ensure_binary integration
pattern that downstream projects (Skyvern PR #5340, crawlee-python
PR #1794, agno PR #8129) already expect.

These helpers let third-party fetchers (changedetection.io plugins,
Crawlee BrowserPool subclasses, agno toolkits) drive
playwright.firefox.launch(executable_path=..., firefox_user_prefs=...)
themselves without depending on the InvisiblePlaywright context
manager owning the lifecycle. Same seed semantics, same humanize
toggle, same extra_prefs overlay as the existing wrapper.

Tests: tests/unit/test_config_public.py adds 14 unit tests covering
deterministic seed, locale/timezone/pin/extra_prefs/humanize
variations, and round-trip via the public namespace. Full unit suite
(392 tests) stays green.

Backwards compatible: InvisiblePlaywright surface is unchanged.
BINARY_VERSION stays at firefox-7. Python-only release.

tests/unit/test_config_public.py

@@ -0,0 +1,125 @@
+"""Unit tests for the public ``config`` helpers."""
+
+import pytest
+
+from invisible_playwright import (
+    ensure_binary,
+    get_default_args,
+    get_default_stealth_prefs,
+)
+from invisible_playwright.config import get_default_stealth_prefs as _direct
+
+
+pytestmark = pytest.mark.unit
+
+
+def test_get_default_args_is_empty_list():
+    """Currently no baseline CLI args, but must return a list (mutable, fresh each call)."""
+    args = get_default_args()
+    assert args == []
+    assert isinstance(args, list)
+    args.append("--foo")
+    # next call must return a fresh empty list, not the mutated one
+    assert get_default_args() == []
+
+
+def test_get_default_stealth_prefs_random_seed_returns_dict():
+    """No seed -> fresh random fingerprint, dict has expected stealth keys."""
+    prefs = get_default_stealth_prefs()
+    assert isinstance(prefs, dict)
+    assert len(prefs) > 0
+    # humanize toggle is always set explicitly
+    assert "invisible_playwright.humanize" in prefs
+    assert prefs["invisible_playwright.humanize"] is True
+
+
+def test_get_default_stealth_prefs_seed_is_deterministic():
+    """Same seed -> byte-identical prefs across calls."""
+    a = get_default_stealth_prefs(seed=42)
+    b = get_default_stealth_prefs(seed=42)
+    assert a == b
+
+
+def test_get_default_stealth_prefs_different_seeds_differ():
+    """Different seeds -> different prefs."""
+    a = get_default_stealth_prefs(seed=1)
+    b = get_default_stealth_prefs(seed=2)
+    assert a != b
+
+
+def test_humanize_false_disables_prefs():
+    """humanize=False removes the maxTime knob and flips the toggle to False."""
+    prefs = get_default_stealth_prefs(seed=42, humanize=False)
+    assert prefs["invisible_playwright.humanize"] is False
+    assert "invisible_playwright.humanize.maxTime" not in prefs
+
+
+def test_humanize_default_sets_max_time_1_5():
+    """humanize=True -> default maxTime is 1.5s, stored as string."""
+    prefs = get_default_stealth_prefs(seed=42, humanize=True)
+    assert prefs["invisible_playwright.humanize"] is True
+    assert prefs["invisible_playwright.humanize.maxTime"] == "1.5"
+
+
+def test_humanize_float_overrides_max_time():
+    """Float for humanize is the explicit cap in seconds."""
+    prefs = get_default_stealth_prefs(seed=42, humanize=3.0)
+    assert prefs["invisible_playwright.humanize"] is True
+    assert prefs["invisible_playwright.humanize.maxTime"] == "3.0"
+
+
+def test_extra_prefs_overlay_takes_precedence():
+    """extra_prefs overlay LAST overrides any baseline value."""
+    prefs = get_default_stealth_prefs(
+        seed=42, extra_prefs={"some.custom.pref": 999}
+    )
+    assert prefs["some.custom.pref"] == 999
+
+
+def test_extra_prefs_can_override_baseline():
+    """A key in extra_prefs that also exists in baseline gets overridden."""
+    baseline = get_default_stealth_prefs(seed=42)
+    a_baseline_key = next(iter(baseline.keys()))
+    overridden = get_default_stealth_prefs(
+        seed=42, extra_prefs={a_baseline_key: "OVERRIDDEN_SENTINEL"}
+    )
+    assert overridden[a_baseline_key] == "OVERRIDDEN_SENTINEL"
+
+
+def test_locale_argument_changes_prefs():
+    """Different locales produce different prefs (Accept-Language affected)."""
+    en = get_default_stealth_prefs(seed=42, locale="en-US")
+    it = get_default_stealth_prefs(seed=42, locale="it-IT")
+    assert en != it
+
+
+def test_timezone_argument_changes_prefs():
+    """Different timezones produce different prefs."""
+    ny = get_default_stealth_prefs(seed=42, timezone="America/New_York")
+    rome = get_default_stealth_prefs(seed=42, timezone="Europe/Rome")
+    assert ny != rome
+
+
+def test_pin_argument_forces_specific_fields():
+    """Pin forces a specific field while the rest stays seed-derived."""
+    plain = get_default_stealth_prefs(seed=42)
+    pinned = get_default_stealth_prefs(
+        seed=42, pin={"hardware.concurrency": 999}
+    )
+    # something in the dict must differ vs the plain seed=42 build
+    assert plain != pinned
+
+
+def test_public_import_matches_direct_import():
+    """Top-level re-export and direct module import return identical output."""
+    a = get_default_stealth_prefs(seed=42)
+    b = _direct(seed=42)
+    assert a == b
+
+
+def test_ensure_binary_is_callable_via_public_namespace():
+    """ensure_binary is re-exported and stays callable from the package root."""
+    # We don't invoke it (would trigger a network download in CI) — just
+    # verify the public attribute is the same callable as the underlying.
+    from invisible_playwright.download import ensure_binary as _direct_eb
+    assert ensure_binary is _direct_eb