Answer sixel handshake queries so apps emit and size sixel correctly

[tig]

Summary

A sixel-capable TUI only emits sixel after a terminal handshake, and the rendered raster is sized from the cell metrics the terminal reports. tuirec answered DA1 but in a form many apps reject, and never answered the window/cell geometry queries ? so apps either never enabled sixel, drew nothing (no screen size), or emitted a raster that overflowed its on-screen cells.

Verified end-to-end: real Terminal.Gui UICatalog Mandelbrot scenario -> tuirec (these fixes) -> sixel-aware agg -> the Mandelbrot renders in the GIF, fitting its view.

What was wrong

• DA1 form. Response was ESC [ ? 62 ; 4 c. Apps that detect sixel by splitting the reply on ; and checking for the token 4 (e.g. Terminal.Gui's SixelSupportDetector: response.Split(";").Contains("4")) saw the token 4c and never enabled sixel.
• No geometry replies. tuirec did not answer CSI 14/16/18 t. Without 18 t (text-area size) Terminal.Gui's ANSI driver could not determine the screen size and drew nothing; without 16 t (cell size) sixel fell back to 1px/cell.
• Raster overflow. When a fixed cell size was reported it did not match agg's, so the app's raster (cells x reported-cell-px) did not map to agg's cells and overflowed the view.

Changes

• DA1 response is now ESC [ ? 62 ; 4 ; 6 ; 22 c (sixel attribute mid-list, as real terminals report it).
• The interceptor answers CSI 16 t (cell size), CSI 18 t (text area in chars), and CSI 14 t (window in pixels).
• The reported cell size is derived from the agg font-size/line-height (row = fontSize*lineHeight, column ~= 0.6*fontSize) so the app's raster matches the cells agg renders.
• Agent guidance (RECORDING-AGENT.md, agent-guide.md) and README.md updated: sixel support, the Linux/macOS-only constraint, --trim=false for alternate-screen apps, and how to record a Terminal.Gui sixel scenario.

Test plan

• go test ./pkg/record/ ? new unit tests cover the DA1 token (TestDA1ResponseAdvertisesSixelAsDelimitedToken) and the geometry replies (TestInterceptorAnswersCellSizeQuery, TestInterceptorAnswersTextAreaQuery); existing sixel pipeline tests still pass.
• End-to-end on Linux: recorded UICatalog Mandelbrot (--args Mandelbrot --trim=false, quit with Esc); cast holds 67+ sixel DCS payloads with Terminal.Gui's palette; the sixel-aware agg renders the Mandelbrot fitting its view.

Known follow-ups (not in this PR)

• --trim discards alternate-screen recordings. With trim enabled (default), a recording whose content is entirely inside the alternate screen is trimmed to nothing; --trim=false is the current workaround. Worth a dedicated fix.
• Windows ConPTY strips sixel DCS (and answers the app's DA1 itself), so sixel cannot be captured on Windows. This is a conhost limitation, not fixable in tuirec; sixel recording is Linux/macOS-only.
• Rendering requires a sixel-aware agg (asciinema/agg sixel support).

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

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