Lay an extension span out as a fixed-width control box (InlineSyntax.inlineBoxWidth)

[dylandeheer]

What

Adds InlineSyntax.inlineBoxWidth: CGFloat?. When set, the extension's span is
laid out as a fixed-width box instead of as text: markers and content collapse,
that many points of line space are kept in their place, and the span behaves as
one control for the caret, the selection, delete, and the pointer.

Why

An extension supplies syntax and content attributes today, which is enough for
a construct that reads as prose but not for one that reads as a control — a
citation playhead, a chip, a token. contentAttributes(theme:) applies to the
whole content range uniformly, so an embedder cannot reserve a width that is
independent of the character count. Without reserved space the only option is
to draw the control over the text, where it overlaps whatever follows and does
not wrap with the paragraph.

Reserved space alone is not enough, though: a span with no glyphs of its own is
a hole in the line. An arrow key stops a dozen times in a spot that never
moves, a click drops the caret inside the construct, one backspace shaves off a
delimiter and turns the box back into raw text mid-sentence, and the pointer
keeps the I-beam over something drawn as a button.

How

• The reserved space rides as kerning on the span's FIRST character, so the box
  is a single advance: the line's height stays the paragraph's rather than the
  box's.
• shrinkInactiveMarkers skips a box span — its shrink would overwrite the
  reserved advance, which sits on a marker character.
• A box's delimiters no longer reveal at the caret. There is no text to reveal,
  and revealing them would shove the box sideways while the user types in the
  line it sits on.
• The auto-link and incomplete-bracket text passes skip box spans, so a
  bracketed construct like [t=12.0-19.5] is not painted as a broken link.
• The styler marks the span with .inlineExtensionBox. NativeTextView
  normalizes every selection through InlineBoxCaret, so the caret rests only
  on the box's edges and a selection takes all of it or none; backward and
  forward delete remove the whole span in one keystroke; and the pointer reads
  the span's laid-out rect as a control and shows the hand, the way the drawn
  task checkbox already does.

Compatibility

nil by default: every existing extension parses, styles, and navigates
exactly as before — covered by a plain-span counterpart in each test. The
registry fingerprint includes the new field, so two registries differing only
in it cannot share cached parses.

Test plan

• InlineExtensionBoxTests: the width lands on the first character, the
  rest of the span collapses, a caret inside the span changes nothing in it,
  a span without the knob keeps upstream layout and caret reveal, the
  fingerprint separates the two registries, and TextKit grants exactly the
  reserved width (same document laid out at two widths).
• InlineBoxCaretTests: arrows step over the box from either side and never
  rest inside it, a dropped caret takes the nearer edge, shift-arrow and a
  partial range take the whole box, backspace and forward delete remove it
  whole, the laid-out rect is the reserved width, and every point of that
  rect reads as a control while points beside it do not.
• Full suite green.

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