Expose the probed cell size (cell aspect) through the public API #154

Not a bug report — an API ask, grown out of the image support in #82.

The problem. Any app drawing proportional content onto a termdom-cellgrid canvas — album art, charts, QR codes, thumbnails — must size its grid against the terminal's real cell aspect (cell height ÷ width in pixels), or the rendering distorts. That aspect is not a constant: the classic 8×16 cell gives 2.0, JetBrains Mono's natural line box gives ~2.2, and line spacing / integer-pixel rounding push real terminals further. On my setup (Konsole + JetBrains Mono) the terminal answers the CSI 16 t query with CSI 6;25;11t — 11×25 px cells, aspect ≈ 2.27 — so a square image fitted at the customary 2:1 renders ~13% too tall, and users resort to hand-tuned magic constants per terminal.

TermDOM already measures this: the framebuffer starts from a guessed 8×16, then negotiateCellPixels() asks CSI 16 t at attach and adoptCellPixels() stores the answer — and intrinsic <img> sizing (the #82 feature) lays images out at round(px ÷ cell) through it. But the measurement is unreachable from app code:

• the package exports map blocks @b9g/termdom/internal/cssom, where getImageCellSize() lives;
• the TermDOM instance, document and window expose no property or event for it, and window.devicePixelRatio returns a hardcoded 1 (fair — CSS px are cells under the unit cell — but it leaves no public access at all);
• the termdom-cellgrid context keeps its cellPixels() behind a private symbol.

The workaround we ship (simplified; the engine sizes a replaced <img> with the probed cell pixels, so a hidden probe leaks the measurement back through public DOM geometry):

// Hidden probe: a 640×200 fully transparent PNG (base64 data URL).
const probe = document.createElement("img");
probe.alt = "";
probe.src = PROBE_PNG_DATA_URL;
probe.setAttribute(
	"style",
	"position:fixed;visibility:hidden;width:auto;height:auto",
);
document.body.appendChild(probe);

// Read the cell aspect back — the box is round(640/cellW) × round(200/cellH).
const box = probe.getBoundingClientRect();
const cellW = 640 / box.width;
const cellH = 200 / box.height;
const cellAspect = cellH / cellW; // ≈ 2.27 once the probe answer lands,
// 2.0 while the 8×16 guess is still standing

Caveats that a real accessor would not have: it costs an image decode and a DOM node per app; precision is bound by the integer rounding of the laid-out box (±0.5 cell per axis — a larger probe narrows it); and it reads the guess until the CSI 16 t answer arrives, so consumers must read it lazily rather than once at startup.

The ask — any one of these would do, smallest first:

1. A getter on the TermDOM instance or its window, e.g. term.cellPixels: {width, height} — the framebuffer's current value, guessed or probed.
2. An accessor on the termdom-cellgrid context (ctx.cellPixels or ctx.cellAspect) — cellgrid consumers are exactly the apps that need it.
3. Export getImageCellSize / getCellPixels from the package root (the exports map currently blocks the internal path).

Two bonus behaviors that would complete it:

• A signal when the value changes — an event, or dispatching a window resize from adoptCellPixels even under the default cellSize: "unit" (today the terminalresize re-probe listener is only installed when measuresCell, i.e. cellSize: "auto").
• Installing that re-probe unconditionally, so a font-size zoom mid-session refreshes image sizing under the default setting too.

Environment: @b9g/termdom 0.2.0, Node 24, primary dev terminal Konsole. All engine internals cited from the 0.2.0 bundle (GUESSED_CELL_PIXELS, negotiateCellPixels, getNaturalSize, getPixelCellSize, the exports map in package.json).