Stop disclosure collapse viewport bounce

[hamzamerzic]

Summary

• reserve a closing disclosure body's height synchronously so bottom-clamped chats do not paint a down/up twitch
• distinguish disclosure taps from real reader scroll gestures while retaining touch-drag ownership
• cover spacer priming, opening behavior, and disclosure input classification

The branch contains #82 as ancestry so its full suite can run in parallel; after #82 lands, this PR naturally reduces to the one follow-up commit.

Review

• reviewed against all ActivityStretch, ToolBlock, and MarkerCard disclosure DOM shapes
• verified the dynamic spacer remains owned by the existing ResizeObserver after the provisional adjustment
• preserved real touchmove ownership and ordinary pointer/touch behavior

Verification

• npm test (1,096 frontend/lib tests + 44 hook tests)
• npm run build
• focused production-tree suite: 62 tests

[hamzamerzic]

Superseded after concurrent landing: main now contains this PR's disclosure-collapse fix as fb22aa9. The remaining stacked drawer/service/test changes were reconciled onto current main in #82, avoiding duplicate commits and conflicts.