RichText.textCustomEmoji: inline animated custom emoji in rich-data bubbles

Add the RichText.textCustomEmoji(fileId:alt:) case end-to-end: model +
Postbox coding, FlatBuffers schema/codec, and Api.RichText
parsing/serialization (lossless), plus display in the InstantPage V2
renderer. The emoji renders as an InlineStickerItemLayer that participates
in the streaming reveal (pops in as the reveal cursor crosses it) and is
gated by the bubble's visibility rect, propagated recursively through the
nested V2 view tree. Also frees the CTRunDelegate extent buffers for the
image/formula/custom-emoji attachment arms and documents the feature in
CLAUDE.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -74,6 +74,31 @@ Spec: [`docs/superpowers/specs/2026-05-19-richdata-streaming-animation-design.md
 - **`Thinking…` header positioning matches TextBubble.** `streamingTextFrame.origin = (bubbleInsets.left - textInsets.left, topInset - textInsets.top)`. `streamingHeaderOffset` = visible bottom + 1pt spacing = where pageView's `frame.origin.y` and statusFrame y-shift attach. Bubble minimum width includes `visible_thinking_width + bubbleInsets.left + bubbleInsets.right + 2`.
 - **Display-link tick re-layouts on extent change.** Tick reads `revealedContentSize` at the new cursor; if the height differs from the previous cursor, calls `requestFullUpdate`. So the bubble grows in flight when the cursor crosses a line/item boundary, not just between chunks. Tick passes `animated: true` to `applyReveal` to fire the snippet pop-in.
 
+## Inline custom emoji (RichText.textCustomEmoji)
+
+`RichText.textCustomEmoji(fileId:alt:)` renders an inline **animated** custom emoji inside rich-data bubbles. Covers API parsing, Postbox + FlatBuffers serialization, and display in the InstantPage V2 renderer; the emoji participates in the streaming reveal above.
+
+### Where things live
+
+| File | Responsibility |
+|---|---|
+| `submodules/TelegramCore/Sources/SyncCore/SyncCore_RichText.swift` | Enum case `textCustomEmoji(fileId: Int64, alt: String)` + Postbox coding (discriminator 17, keys `ce.f`/`ce.a`), `==`, `plainText` (returns `alt`), and FlatBuffers codec. |
+| `submodules/TelegramCore/FlatSerialization/Models/RichText.fbs` | FlatBuffers schema — `RichText_CustomEmoji` union member + table. **Source of truth**; the Bazel `flatc` genrule regenerates `*_generated.swift` at build time (the checked-in `Sources/*_generated.swift` is stale). |
+| `submodules/TelegramCore/Sources/ApiUtils/RichText.swift` | `Api.RichText.textCustomEmoji` ⇄ Swift, lossless both ways. |
+| `submodules/InstantPageUI/Sources/InstantPageTextItem.swift` (`attributedStringForRichText`) | Emits a single placeholder char carrying `ChatTextInputAttributes.customEmoji` (a `ChatTextInputTextCustomEmojiAttribute`) + a `CTRunDelegate` sized `font.pointSize · 24/17`. |
+| `submodules/InstantPageUI/Sources/InstantPageV2Layout.swift` (line-breaker) | Collects per-line `InstantPageTextLine.emojiItems`; overwrites each placeholder char's `characterRect` with a full cell (`width = itemSize`) so it feeds the reveal cost map. |
+| `submodules/InstantPageUI/Sources/InstantPageRenderer.swift` (`InstantPageV2View`) | Owns the `InlineStickerItemLayer`s: `updateInlineEmoji` (create/reuse/remove/position), `updateEmojiReveal` (reveal-driven pop-in), `updateEmojiVisibility` + `propagateVisibilityRect`. Layers attach to each text view's `emojiContainerView`. |
+
+### Non-obvious invariants
+
+- **flatc casing/`required` gotchas.** Edit `RichText.fbs`, not the generated Swift. Scalars (`long`) cannot be `(required)` — only strings/tables can. A union member `RichText_CustomEmoji` generates the Swift enum case `.richtextCustomemoji` (everything after the suffix's first letter is lowercased); the table type stays `TelegramCore_RichText_CustomEmoji` and field accessors keep `.fbs` casing (`value.fileId`). See the `flatbuffers-codegen` memory.
+- **`ChatTextInputTextCustomEmojiAttribute` is reused end-to-end** (display layer ⇄ layout model). The attribute is written to the placeholder in `attributedStringForRichText` and read back by the V2 line-breaker under the SAME key (`ChatTextInputAttributes.customEmoji`); `InlineStickerItemLayer.init` consumes it directly and resolves the file lazily from `fileId`.
+- **Emoji participates in the streaming reveal.** Its placeholder char's `characterRect` is overwritten to a full cell (width = `itemSize`, baseline-relative bottom at `y=0`), so the width-based cost map charges it like other content. `updateEmojiReveal` pops the layer in (alpha 0→1 + scale) when `charIndexInItem < currentRevealCharacterCount`; unrevealed → opacity 0.
+- **Layers sit ABOVE the reveal mask.** They attach to `InstantPageV2TextView.emojiContainerView` (a sibling above `renderContainer`), NOT inside it — so the reveal mask wipes glyphs while emoji pop in independently. Adding a CTRunDelegate-glyph to the mask would clip-wipe them instead.
+- **Layers are owned by `InstantPageV2View`, not the text view.** Keyed by `InlineStickerItemLayer.Key(id: fileId, index: occurrence)`. The pageView is rebuilt per `stableVersion` bump (see streaming section), so the dict starts fresh each chunk — no orphan/leak across rebuilds; within one view, stale keys are pruned.
+- **`visibilityRect` gates looping; `nil` means "not visible".** The bubble's `visibility` override pushes a full-width sub-rect to the root `pageView.visibilityRect`, re-pushed in the apply closure after `pageView.frame` is set (because `streamingHeaderOffset` shifts across chunks without a `visibility` change). `propagateVisibilityRect` converts the rect into each nested V2View's coordinate space (`self.convert(_:to:)`) for details bodies / table cells+title, fanning out via each child's `didSet`.
+- **CTRunDelegate extent buffers must be freed.** Every inline-attachment arm (`.image`/`.formula`/`.textCustomEmoji`) in `attributedStringForRichText` allocates an `extentBuffer`; the `dealloc` callback must `deallocate()` it (it re-runs per layout pass).
+
 ## Postbox → TelegramEngine refactor (in progress)
 
 A gradual migration is underway to eliminate direct `import Postbox` from consumer submodules in favor of `TelegramEngine`.