fix: split stealth init scripts by browser engine and add Firefox stealth

The stealth system previously injected all 12 init scripts unconditionally
into every browser engine. Chromium-specific scripts (window.chrome stubs,
ANGLE WebGL strings, CDP cleanup, HeadlessChrome UA strip) were no-ops or
actively suspicious on Firefox, while Firefox-specific headless vectors
were unaddressed.

Split stealthInitScripts into three categories:
- stealthCommonScripts (4): webdriver, outerWidth/Height, permissions, Notification
- stealthChromiumScripts (8): existing Chromium-specific scripts
- stealthFirefoxScripts (5): new Firefox-specific stealth:
  - navigator.webdriver getOwnPropertyDescriptor hardening
  - WebGL renderer spoof with Mesa/Intel strings
  - mozInnerScreenX/Y non-zero spoof
  - navigator.hardwareConcurrency normalization
  - PDF.js plugin list override

browser_init.go now selects common + engine-specific scripts based on
opt.Browser. Tests updated with per-category validation and cross-
contamination checks.

Closes #69

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browser_init.go

@@ -61,7 +61,13 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		if opt.Browser == BrowserChromium {
 			launchArgs = append(launchArgs, stealthChromiumArgs...)
 		}
-		initScripts = append(initScripts, stealthInitScripts...)
+		initScripts = append(initScripts, stealthCommonScripts...)
+		switch opt.Browser {
+		case BrowserChromium:
+			initScripts = append(initScripts, stealthChromiumScripts...)
+		case BrowserFirefox:
+			initScripts = append(initScripts, stealthFirefoxScripts...)
+		}
 	}
 
 	launchArgs = append(launchArgs, opt.LaunchArgs...)