feat(client): add read-only terminal region queries

[schickling-assistant]

Why

Related to #134. Embedded browse, pan, and scan surfaces need a stable view of
the daemon's terminal model without attaching a writable client or changing the
shared PTY geometry.

This is a read-only alternative to #135 for those surfaces. It does not replace
the streamed effective-geometry approach for realtime writable clients.

What

• Add a bounded terminal-region request/response to the socket protocol.
• Add queryTerminalRegion() to the programmatic client API.
• Return daemon generation, terminal revision, effective geometry, active-buffer
  metadata, cursor and mode state, and structured cells.

How

The daemon queues each query behind an xterm write callback, then copies the
requested region synchronously. This makes generation, revision, geometry, and
cells describe one terminal-model state without timing heuristics.

Query sockets are read-only, never send ATTACH, and have no requested terminal
geometry. They therefore do not participate in min-wins negotiation, trigger
SIGWINCH, or update attach metadata.

Colors use tagged default, palette, and rgb values. Requests are capped at
100,000 cells and responses retain the protocol's 32 MiB packet limit.

Scope and limits

• Polling only; no realtime subscription in this change.
• Bounded regions can scan retained scrollback, but there is no one-shot
  unbounded scrollback response.
• The response reflects xterm-headless's terminal model, not renderer output.
• Palette indexes are preserved, but palette definitions are not.
• OSC 8 metadata, graphics protocols, fonts, and glyph rendering are out of
  scope.
• Client geometry detail remains the responsibility of stats APIs.

Verification

• npm run typecheck
• npm run build
• Focused protocol, integration, and client tests: 7 passed
• Full test/docs gates and remote CI are pending while this PR remains draft

Posted on behalf of @schickling

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[schickling-assistant]

Closing this draft because its protocol IDs collide with active upstream protocol work (#131 ACTIVITY and #133 GUARDED_DATA). The unsupported-server behavior and overlap with the evolving revision contract also need an explicit capability/version design before this can be proposed safely.

Preserving the branch and commit as investigation evidence; no code is being deleted.

Posted on behalf of @schickling

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agent_runtime	Codex CLI 0.145.0
agent_model	unknown
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