Add a new detectContentBlocked() function that detects when sites serve
pages but block actual content from headless browsers. Uses five ordered
heuristics (OR logic): provider blocked selectors, provider blocked text
patterns, empty content areas, generic error text with short body, and
persistent loading indicators. Exports CONTENT_BLOCKED_TEXT_PATTERNS.

Expand the addInitScript block with additional stealth measures:
- Spoof window.chrome object (present in real Chrome, missing in headless)
- Spoof navigator.plugins with non-empty PluginArray-like object
- Set navigator.languages to ['en-US', 'en']
- Override WebGL vendor/renderer to Intel Inc. / Intel Iris OpenGL Engine
- Override permissions.query for 'notifications' to return denied state

Import detectContentBlocked and add matchProviderByDomain helper with
lazy-loaded Map for O(1) provider lookup. After goto navigation and
waitForLoaded, detect content blocking using provider-specific config
from providers.json. When detected, add contentBlocked, warning, reason,
and suggestion fields to the result. Add --no-content-block-detect flag
to skip detection.

Add 19 tests covering all detection heuristics:
- Provider-specific blocked selectors and text patterns
- Empty content area detection with threshold
- Generic error text with short body
- Persistent loading indicators (visible vs invisible)
- Error handling for page.$() and textContent() failures
- Default emptyContentThreshold of 200
- X.com-specific: empty feed, error state, no false positives
- CONTENT_BLOCKED_TEXT_PATTERNS export validation

…xports

Add two tests to the existing auth-wall-detect test suite confirming
that the new detectContentBlocked function and CONTENT_BLOCKED_TEXT_PATTERNS
constant are properly exported from the module.

…e case tests

- Cache bodyText fetch in detectContentBlocked to avoid redundant DOM query
- Export LOADING_INDICATOR_SELECTORS for testability
- Add empty contentSelectors array edge case test
- Add LOADING_INDICATOR_SELECTORS validation tests