Scale box button brackets to the content height

[LuciusChen]

Problem

Box button brackets are SVG images of exactly 1 char height. When the
button body or another glyph earlier on the same line renders taller than
one default-font line — color emoji glyphs (e.g. Apple Color Emoji has
noticeably larger ascent/descent than monospace fonts), CJK fonts, custom
emoji / avatar images — the screen line grows, but the brackets stay 1
char high. Reactions and inline keyboard buttons end up with content
sticking out above and below the button "pill". The same issue affects
ordinary UI buttons such as Download when a taller media icon precedes
the button on its line.

Before

Approach

This PR introduces a content value for the :height bracket property:

(reaction
 :left-bracket ("(" :width 0.75 :height content :rx 0.5)
 :right-bracket (")" :width 1.0 :height content :rx 0.5 :margin 0.25)
 ...)

With :height content, bracket height and baseline are measured from
the real rendering of the current line prefix and button body
(telega-box-button--content-metrics):

1. Line height is measured with buffer-text-pixel-size in a work
   buffer. Measuring the actual redisplay result (instead of guessing via
   per-character font-at, which bypasses emoji compositions such as
   VS16/ZWJ sequences) makes emoji compositions, embedded images (custom
   emoji, sender avatars) and (raise ...)-displayed text all "just
   work". An anchor char in the default face is appended, since screen
   line metrics are max(ascents) + max(descents) over all glyphs on the
   line.

2. Baseline position is recovered with a probe: appending an
   :ascent 100 space of the measured line height grows the line by
   exactly the line's descent, so ascent = 2*H1 - H2. The resulting
   :ascent percentage is ceiling'ed — image ascent is truncated as
   floor(percent * height / 100) on display, so rounding down here
   would place the bracket 1px lower, with content peeking out above the
   button.

3. Geometry. The SVG viewport stays 1 char high and is
   proportionally scaled up to the displayed :height, so bracket caps
   keep their shape, growing wider for taller content (Telegram-like pill
   look). Two pixel-alignment details:

   • the proportional scale is compensated in the SVG outline width, so
     the outline is exactly :outline-width pixels on display;
   • the mask hole is inset by exactly the outline width (previously it
     was inset by ~1.5x due to half-pixel offsets plus the hole's own
     stroke).

   Both make the bracket's visible background and outline pixel-aligned
   with the button body, where the horizontal outline lines are drawn by
   the :box of telega-box-button--body-face.

Since the left bracket is inserted before the body is known,
telega-ins--with-style re-generates the left bracket image after the
body has been inserted and replaces its display property. Generated
images are cached with the computed height/ascent as part of the cache
key.

:height content is enabled for the reaction (and, via inheritance,
reaction-chosen / reaction-paid), telega-ui (including comments
and file actions such as Download), iv and keyboard-* styles.

After

Compatibility

• On Emacs without buffer-text-pixel-size (< 29.1) metrics return nil
  and brackets fall back to the previous fixed 1 char height — no
  behavior change.
• Styles that do not declare :height content (e.g. default) are
  unaffected: for height = 1 all geometry reduces to the previous
  formulas.
• Plain-text content measures exactly 1 char high, so buttons with
  ordinary labels look the same as before.

Testing

• 3 new ERT tests (telega-box-button-content-metrics,
  telega-box-button-content-height-inserter,
  telega-box-button-bracket-metrics) cover the metrics math (including
  the ceiling ascent regression), same-line prefix preservation, left
  bracket re-generation, telega-ui inheritance and generated image
  specs. Full suite passes: 26/26.
• Rendering verified interactively across font sizes (PragmataPro Mono
  10/14/16/20 + Apple Color Emoji + CJK font): bracket background
  extents are pixel-identical to the button body, measured from actual
  window captures.

🤖 Generated with Claude Code

[zevlg]

This looks very promissing! let me first check this locally before merging it in

[LuciusChen]

I've tested all the scenarios shown in the screenshots above, and they now work as expected.

[LuciusChen]

Added an explicit :background nil to SVG bracket images so their transparent area can follow cursor-face highlighting instead of retaining the passive background. The full test suite still passes (26/26).