spec: apply outcome-not-technique rule to existing pages

Judged all pages by the scope rule added after PR #82 (auditable website
outcome, not build technique):

- Downgrade View Transitions, invoker commands, and visibility-aware
  rendering from recommended to optional — real platform features, but
  context-dependent polish, below the 'a modern site should do it' bar.
- Remove entry-exit-animations and field-sizing — CSS features whose
  benefit is authoring convenience; nothing for a visitor, crawler, or
  agent to gain and nothing to audit.
- Clean relatedSlugs on popover-api, reduced-motion, invoker-commands;
  unlink the now-dead links in the 1 July changelog entries.
- Changelog: one status entry, one removed entry.
- Regenerate OG images (count-driven + status badges on the three
  downgraded pages); delete the two per-page images.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jdevalk

public/og-default.png

@@ -5,4 +5,4 @@ type: added
 relatedSlugs: [entry-exit-animations]
 ---
 
-Added a page on [entry and exit animations](/spec/foundations/entry-exit-animations/) — the `@starting-style` rule and `transition-behavior: allow-discrete`, which let elements animate as they appear and disappear, including to and from `display: none` and the top layer, without JavaScript timers. Documented as `optional`: it applies only when you choose to animate, and has been Baseline across major browsers since 2024.
+Added a page on entry and exit animations — the `@starting-style` rule and `transition-behavior: allow-discrete`, which let elements animate as they appear and disappear, including to and from `display: none` and the top layer, without JavaScript timers. Documented as `optional`: it applies only when you choose to animate, and has been Baseline across major browsers since 2024.