docs: revise ADR 0004 with Selenium 4 evidence and Option 2 recommendation

Replace dual-track recommendation with full Playwright migration plus
time-bounded Selenium deprecation period. Adds:

- Empirical Selenium 4 shadow DOM verification: 6-hop nesting, 452 shadow
  roots per page, 14-line traversal vs 1-line Playwright equivalent
- Effort comparison from PoC (~25x lines/min on tested surface, with
  sample-size caveat that harder keywords likely 3-5x)
- Per-release maintenance trajectory comparison (Selenium worsening,
  Playwright effectively flat)
- 5-phase migration path: unblock Playwright, port keywords, compat
  shim, downstream coordination, deprecation/removal
- Selenium 4 added as Option 4 with empirical evidence showing it does
  not solve the shadow DOM problem at scale

Verification scripts and findings stored under
docs/superpowers/evidence/2026-04-27-robot-poc/ for reproducibility.

Author: jstvz