✨ [Story analytics] Add caption state analytics events

[michaeldegori]

✨ [Story analytics] Add caption state analytics events

Summary

Adds analytics dispatch for the existing StateProperty.CAPTIONS_STATE so publishers can track when users toggle captions on or off, mirroring the existing STORY_MUTED / STORY_UNMUTED events.

Why

Audio mute/unmute already emits analytics:

// extensions/amp-story/1.0/amp-story.js
this.storeService_.subscribe(
  StateProperty.MUTED_STATE,
  (isMuted) => {
    this.analyticsService_.triggerEvent(
      isMuted
        ? StoryAnalyticsEvent.STORY_MUTED
        : StoryAnalyticsEvent.STORY_UNMUTED
    );
  },
  false /** callToInitialize */
);

Captions are a parallel user-toggleable media state (.i-amphtml-story-captions-button paired with .i-amphtml-story-no-captions-button) but have no analytics surface today. Publishers can't measure caption engagement without it.

Changes

• story-analytics.js — adds two values to the StoryAnalyticsEvent enum:
  • STORY_CAPTIONS_ON: 'story-captions-on'
  • STORY_CAPTIONS_OFF: 'story-captions-off'
• amp-story.js — subscribes to StateProperty.CAPTIONS_STATE next to the existing mute-state analytics subscriber. Uses callToInitialize: false so the default-on initialization does not fire an event; only user toggles do.
• amp-story-analytics.md — documents the two new triggers in the same voice as the existing "Mute trigger" / "Unmute trigger" entries.

Naming notes

• Event-name verb: on / off matches the captions-on host attribute on the system layer (amp-story-system-layer.js) and reads more naturally than enabled / disabled. Audio uses muted / unmuted, captions uses on / off, so each follows its own native vocabulary.
• Callback parameter captionsState matches every other CAPTIONS_STATE subscriber in the codebase (amp-story-page.js, amp-story-system-layer.js) and the store-service typedef.

Out of scope (intentional follow-up)

• No AnalyticsVariable.STORY_HAS_CAPTIONS parallel to STORY_IS_MUTED. Publishers can distinguish state via the new event names; if reviewers prefer a variable for cross-event reporting parity I'm happy to add it in a follow-up.
• No new test. The audio dispatch in amp-story.js is also untested (mirrored in test-analytics.js and test-amp-story.js). Happy to add coverage for both if preferred.

Test plan

• npx eslint clean on touched files
• amp unit --files=extensions/amp-story/1.0/test/test-analytics.js — all 4 existing tests pass
• Manual smoke: toggle captions in a story with <amp-story-captions> and an amp-analytics listener on story-captions-on / story-captions-off

Semantics check

StateProperty.CAPTIONS_STATE defaults to true in the store. With callToInitialize: false, the first emission is the first user toggle, so an opening-tap that hides captions correctly fires STORY_CAPTIONS_OFF (true → false). This matches the audio precedent's semantics.

[michaeldegori]

Superseded by upstream PR ampproject#40505.