File: optional width/height for visual files

[EgorPPS]

Two optional integer properties on File: width and height, in pixels.

Why

A file's pixel size is a property of the file. Any application laying out an
image needs it before the bytes arrive — otherwise it reserves a guessed box
and corrects it on decode, which moves everything already on screen.

We hit this as a concrete defect in Meshenger. Scrolling back through a chat of
photographs moved the reader by hundreds of pixels per screen; the worst single
row shifted 330 px, because the box was reserved from a per-device guess (aspect
1.2) while the picture was 0.46. The user's words were "очень много дерганий и
автопрокручиваний".

Our first fix put the dimensions into our own Message envelope, and that is
why this PR exists rather than a changelog entry. A photo editor or a video
editor reading the same data has no reason to look inside a messaging ontology
for a picture's size — it would re-derive it per device, or never find it.

Why not MediaAsset

MediaAsset already carries width/height, which suggests the ecosystem does
want these two numbers. But it requireds observationRef, binding them to an
Observation. A photograph sent in a chat has no observation, and inventing one
to carry two integers would be worse than the problem it solves.

If you would rather these live only on MediaAsset, we would need a way to use
it without an observation — happy to send that PR instead.

Why it has to come from the schema

File is "additionalProperties": false. No application can add these
privately without its envelopes failing strict validation in any reader that
enforces the schema. (We have been bitten by precisely that on another strict
ontology, so our File writes stay inside the registered field set.)

Compatibility

Purely additive. Both optional, required unchanged, no existing field touched.
A reader that does not know them behaves exactly as today; a writer that does
not measure omits them. Range is minimum: 1 — a zero-pixel image is not a
thing, and allowing 0 would let a bad writer reserve an empty box.

Follow-up, if this shape is welcome

We carry other file-describing facts in the wrong envelope for the same reason
(how the phone was held when a photo was taken, which recording a frame belongs
to and at what offset). We would rather propose those here, one concern at a
time, than keep a private dialect of a shared schema.

[coderabbitai]

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