Research Edition: classification signal varies drastically by platform/browser — document and narrow the gap

Study-design concern surfaced during v0.14.0b5-research dogfooding:

Browser categorization quality depends on what URL/title signal each platform+browser exposes, and the variance is large:

Setup	Signal	Quality
macOS + Chrome/Safari	full URL via Apple Events	strongest
macOS + Firefox	URL via AX tree (gecko walk)	strong
Firefox + Add URL to Window Title extension	domain embedded in title	good, cross-platform
Firefox / others, no extension	title keywords only	weak
Windows (any browser)	title only — no URL inspection path in the watcher today	weakest

Implications: (1) macOS-Chrome participants produce systematically better-categorized data than Windows participants — a per-platform bias in the study dataset worth documenting in the study methodology; (2) the participant guide could recommend the URL-in-title extension for Firefox users as a cheap signal booster; (3) longer-term, Windows Chromium exposes the address bar via UIAutomation — a candidate enhancement for aw-watcher-window to narrow the gap.

Filed as follow-up work, not a release blocker.