refactor(flterm): consolidate renderer ownership

[adonm]

Summary

• make TerminalRenderPipeline own frame buffers, painters, atlas state, Kitty state, paint ordering, and terminal dirtiness
• delegate terminal defaults, cursor-key encoding, and size reporting to libghostty or cached layout state
• remove one-use render/widget adapters while preserving the public TerminalView behavior

Why

This removes duplicate ownership and synchronization paths from flterm. The adapter remains responsible for Flutter lifecycle and presentation while libghostty remains authoritative for terminal state.

This is an independent extraction from #101, replayed directly on current main; it has no dependency on the other extracted PRs.

Verification

• flutter test with the supported prebuilt native-asset provider: 686 passed
• dart format --output=none --set-exit-if-changed packages/flterm: clean
• dart analyze packages/flterm: only the 3 existing info-level collection-style lints

[adonm]

Superseded by the upstream controller/view refactors. Closing.

What this PR proposed — consolidate frame buffers, painters, atlas state, Kitty state, paint ordering, and terminal dirtiness in the render pipeline — is now the upstream structure:

• refactor(flterm): separate controller and view responsibilities #143 (bf6ea62) established the controller/view boundary; refactor(flterm): align internal component names with domain boundaries #144 (6deaece) aligned the internal component names.
• RenderPipeline now literally documents Owns the frame buffers, frame builder, and paint stack for one render box, with TerminalRenderBox retaining only widget/render-object lifecycle.
• The one-use adapters this PR inlined no longer exist in the current structure.

Happy to reopen a focused PR if a specific ownership gap remains after #143/#144 — otherwise this is covered.

[adonm]

Closing as superseded; see the comment above for the upstream commits that cover this.