docs(changelog): add 2026-04-20 weekly summary + regenerate RSS
Covers this week's work: core consolidation (cmd/page/hud merged into
the core background renderer + ext:all-loaded race fix), the Phase
3.11 preload.js deletion sweep, single-tile feature collapses
(groups/tag-actions/lex/entities/websearch), pubsub broadcaster fixes
(webview guests, full-URL echo prevention, window-entry registration),
test parallelization (smoke split, desktop-serial suite, per-describe
isolation), and a handful of one-off fixes.