feat(browser-actions): annotate step for instructional recordings

[chubes4]

Fixes #2548.

New annotate step kind that renders into an isolated overlay: its own shadow root, pointer-events: none, top z-index, re-anchored on scroll and resize. It doesn't need the evaluate grant.

• Shapes:
  • highlight (ring / box / underline)
  • spotlight (dims the viewport except the target's padded box)
  • arrow (SVG, placed by direction, with an optional draw stroke animation)
  • label (anchored to a target)
  • caption (position: top|center|bottom)
• Every annotation takes optional id, durationMs (auto-clear), and animate (draw|fade|none). clear: [ids] | true removes them.
• Run-level theming via annotation-theme-json=<object|@file>: accentColor, textColor, background, fontFamily, fontSize, strokeWidth, radius. Neutral defaults, no per-step theme.
• position is one shared field typed "top"|"center"|"bottom". scroll (feat(browser-actions): visible pointer, human-paced motion and smooth scroll step #2550) still accepts only top/bottom, and caption accepts all three. The issue's from was renamed to direction to avoid colliding with drag.from.
• README documents it.

Tests

npm run build clean.

• browser-annotations.browser.test.ts 2/2: shape/clear validation, plus an overlay that doesn't intercept clicks, follows scrolling and clears by id
• test:browser-video-capture 4/4
• presentation validation 2/2 and browser 1/1
• browser-preview-routing 16/16

Follow-ups (not blocking)

• The browser test doesn't yet assert the theme argument, the SVG arrow geometry, or the spotlight cutout box.
• The run theme is held in module state set per run (setBrowserAnnotationTheme). Threading it through the step executor would be cleaner for concurrent sessions.

Drafted by Homeboy cooks (cook-detached-fa0ce1e3, cook-detached-282c847a, cook-detached-ea28c3dd). Gates passed, but publication stalled (promotion_operation: operation_in_progress). With Chris's authorization, the annotation changes were applied onto current main (after #2549 and #2550), re-verified, and published outside Homeboy.

AI-assisted: drafted by an OpenCode agent, reviewed and finalized by Extra Chill Bot.