feat(export)!: replace on-screen bitmap capture with headless PNG exporter

[akshay2211]

Remove the legacy screen-capture PNG route and promote the headless raster exporter as the sole PNG path.

Removed (BREAKING):

• DrawBoxController.saveBitmap()
• Event.PngSaved / Intent.SaveBitmap
• capturePending/captureLayer machinery + State.invokeBitmap in DrawBox

Added / promoted:

• DrawBoxController.exportPng(scale, background, textMeasurer) -> Event.PngExported(bytes)
• PngExporter + per-platform PngEncoder (renders state.elements off-screen via the SDK's real renderElement, then encodes to PNG bytes)

Sample app: onDownloadPng now calls exportPng(...) and ImageSaver.savePng takes ByteArray (writes the already-encoded bytes directly, no re-encode).

Docs: migration guide section, CHANGELOG, README, and the api-reference / faq / features / architecture pages updated. Warrants a 3.0.0 release.

(cherry picked from commit 04999a7)

Summary

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking, additive to the frozen 2.x surface)
• Incubating API (opt-in via annotation)
• Breaking change (would land in a future major)
• Docs / build / CI only

Affected targets

• Android
• iOS
• JVM Desktop
• Web (WASM)
• Web (Kotlin/JS)

Public API impact

• No public API change
• Additive only (new symbols, no signature changes)
• Modifies a 2.x frozen surface — link to RFC or discussion:

Testing

• Unit tests added / updated
• Snapshot tests updated (Roborazzi baselines regenerated if applicable)
• Manually verified on:

Checks

• ./gradlew :DrawBox:spotlessCheck passes
• ./gradlew :DrawBox:jvmTest passes
• ./gradlew :DrawBox:dokkaGenerateHtml passes
• CHANGELOG entry added under the correct release heading (if user-facing)
• Migration notes added (if the JSON scene format or a frozen surface moved)

Screenshots / recording

Linked issues