Report Avalonia cell pixel dimensions so server-sized MXP images retain aspect ratio #124

Verified reproduction

Follow-up to merged #122. That change fixes dimensions calculated locally but correctly preserves explicit MXP W/H character dimensions. SharpMUSH lays out its default CONNECT logo in a 33-column figure and sends both dimensions, so it also needs the client's actual cell geometry.

An isolated test using SharpMUSH's actual rendering worker and default 512×512 assets/logo.png produced:

• Unknown cell pixels: <IMAGE ... W=33c H=17c> (server fallback 10×20).
• Reported 10×24 cells: <IMAGE ... W=33c H=14c>.
• Reported 16×40 cells: <IMAGE ... W=32c H=13c>.

On a 10×24 client the unknown-metric box renders as 330×408 pixels, rather than approximately square. Correct geometry gives 330×336, within whole-cell rounding.

Cause and scope

SharpMUTerm sends NAWS character dimensions but does not answer the server's CSI 16 t cell-pixel query. Its text parser discards non-SGR CSI sequences, and the telnet session currently has no pixel-query responder. SharpMUSH's pinned TNC 4.8 already accepts CSI 6 ; height ; width t and carries those dimensions into initial and subsequent render capabilities.

The Avalonia scrollbar is an overlay, not a deducted layout gutter. A separate runtime pane/NAWS comparison matched dimensions, including overflowing scrollback. This issue is about missing pixel geometry, not image stripes. The reproduction establishes the bug independently; it does not establish which exact metrics or build produced the user's screenshot.

Expected fix and verification

Answer cell-size queries with current measured geometry at the telnet byte-stream layer, before line buffering; refresh geometry when zoom/font metrics change. Keep explicit server cell reservations intact. Test fragmented queries, no-newline negotiation, unavailable metrics, later metric updates, and server rendering. No SharpConsoleUI changes are needed.