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.

July 2026 update: reworked onto current main

This branch has been rebased onto main after the controller/view boundary refactors (#143, #144) and reworked for the current frame-source pipeline:

• FrameBuilder.semanticsText() walks the current RowIterator/CellIterator state; RenderPipeline forwards it.
• TerminalRenderer/render box capture the snapshot after frame sync when a semanticsGeneration advances.
• TerminalView owns the generation bumps (terminal output, primary-screen scroll, controller swap) and the ValueNotifier-driven semantics node.

Verification

• Full flterm suite with source-compiled native assets (Zig 0.16): 983 passed, including the new frame-builder semantics test and four view tests (non-concealed text, label delegation, scrollback viewport tracking, controller swap clearing).
• dart format --set-exit-if-changed packages/flterm and dart analyze packages/flterm: clean (only 2 pre-existing info lints under tool/benchmarks).

Carried downstream in the Zuko remote-shell client since flterm 0.0.4; this is the port onto the refactored 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).