feat(flterm): expose accessible terminal semantics

[adonm]

Summary

Adds a screen-reader semantics node to TerminalView:

• semanticsLabel / semanticsHint params with sensible defaults ('Terminal' / 'Activate to focus terminal input'). Set semanticsLabel: null to delegate the accessible surface to an embedding application.
• The semantics node exposes the visible, non-concealed terminal viewport as plain text (wide cells widened, SGR 8 invisible cells omitted, wrapped rows joined), captured from the render pipeline after each paint and coalesced through a bounded 100 ms refresh interval — so output updates never announce as a live region.
• The snapshot follows the visible scrollback viewport (primary screen only), and switching controllers clears the cached value.
• Focus/tap actions route to terminal input focus.

Implementation

• FrameBuilder.semanticsText() walks the existing RowIterator/CellIterator state and emits plain text; RenderPipeline/renderer forward a monotonically increasing semanticsGeneration and an onSemanticsText callback fired from paint after frame sync.
• _TerminalViewState schedules generation bumps on terminal output and primary-screen scroll, and rebuilds the semantics node through a ValueNotifier without recreating the terminal subtree.

Tests

• Frame builder: semantics iteration preserves subsequent terminal updates.
• View: visible non-concealed text + hint/actions; label delegation via null; scrollback viewport tracking; controller swap clears cached semantics.

This was carried downstream in the Zuko remote-shell client and is ported here from that implementation onto the refactored frame-source pipeline.

[adonm]

Local validation on the pinned Flutter beta: flutter analyze clean, and the full rendering/view/controller test suites pass — 540 tests, including the four new semantics tests (visible non-concealed text, label delegation, scrollback viewport tracking, controller swap clearing).