Report terminal pixel size so Kitty/icat pixel probes succeed

[phall1]

Problem

kitty +kitten icat aborts before sending any image with:

Terminal does not support reporting screen sizes in pixels, use a terminal such as kitty, WezTerm, Konsole, etc.

icat preflights the terminal for PIXEL dimensions. A phux pane reported 0x0: the PTY winsize ws_xpixel/ws_ypixel and the XTWINOPS CSI 14 t reply both derived their pixels from a per-cell size that stayed (0, 0) until a client announced a viewport with usable pixel metrics. Most clients report cells but not pixels, so the geometry stayed zero and the probe failed.

Fix (pixel-size REPORTING only)

Seed a fallback per-cell pixel size DEFAULT_CELL_PX = 8x16 so the derived pixel geometry is never zero. Cells (cols/rows) remain authoritative; pixels are always cells x cell size. A real client's reported cell size still wins and stays sticky across pixel-less resizes (e.g. an agent TERMINAL_RESIZE).

Applied at the three reporting sites in phux-server:

• spawn-time openpty winsize — a child that reads TIOCGWINSZ before any client resize (icat preflighting at shell startup) now sees nonzero pixels.
• initial size_report shared with libghostty's on_size callback — CSI 14 t answers nonzero even before the first resize.
• handle_resize — already derives the winsize ioctl and size reports from the sticky cell size, now nonzero.

The in-band CSI 14 t / CSI 16 t / CSI 18 t path is already wired through libghostty's on_size callback (exercised by xtwinops_size_queries_answered_from_resized_geometry); it simply needed a nonzero cell size to report, which this change guarantees.

Server-local; no wire change — no spec update or version bump required.

Scope

This is the pixel-size REPORTING fix only. Full Kitty graphics RENDERING is a separate deferred item with its own ADR and is not attempted here.

Tests

• New winsize_pixels_default_when_no_client_reports_metrics: with no client ever reporting pixel metrics, asserts spawn-time winsize is 80x24 cells -> 640x384 px and a pixel-less resize tracks 100x40 -> 800x640 px.
• Existing resize_with_cell_px_updates_pty_winsize_pixels and xtwinops_size_queries_answered_from_resized_geometry continue to pass (client-reported cell size still wins).

just ci green locally.

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