Measure the terminal's cell on request, and announce a new one

[brainkim]

For #154: programs drawing proportional content, like images, charts or QR codes on a termdom-cellgrid canvas, need the terminal's real cell size in pixels. TermDOM already asks the terminal for it but kept the answer to itself.

API

• term.measureCell() asks the terminal for its cell in pixels (XTWINOPS 16) and resolves to {width, height}. It resolves to null when there is nothing to ask (before attach(), after dispose(), output not a terminal) or the terminal doesn't answer. Calls made while a question is out share its answer. It reports the terminal's measurement whatever the cellSize option says, so it gives the real cell under the default unit cell, where it differs from the page's 1×1.
• cellchange fires on the TermDOM with width and height when the cell is measured at a different size: when the first answer after attach() replaces the size assumed until then, and after a font zoom. The event class is CellChangeEvent, exported with CellChangeEventInit.

The engine's internal 8×16 guess never reaches the public API.

Fix

The cell was asked again after a terminal resize only under cellSize: "auto". Under the default unit cell, a font zoom left images at the size the old cell gave them. It is now asked after every resize.

Tests and docs

• tests/cell-size.test.ts: the answer, the null cases, shared questions, and cellchange at attach and after a zoom.
• tests/img.test.ts: a font zoom resizes an image under the unit cell. This test fails on main.
• The API guide documents measureCell() and cellchange, and the images guide mentions both.

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