fix(web): reserve one scrollbar gutter across the composer's text layers

Second review round escalated the wrap-width divergence from a separate
concern to a defect in this fix's own premise, and it is right.

Only .input scrolls, so only .input loses content width to a scrollbar that
consumes layout space — what Windows and Firefox draw, and what the theme's
global `::-webkit-scrollbar` width makes chromium treat as occupying space.
A narrower .input wraps a long soft-wrapped draft onto more lines, so it
grows taller, its scroll maximum exceeds the backdrop's, and the mirrored
offset clamps below the caret. That is the same failure the trailing-line
sentinel fixes, in the same direction, so deferring it would have shipped a
fix that does not hold where users run a classic scrollbar.

My first attempt to reproduce it found nothing and was wrong: the probe
content was not wrap-sensitive. With varied-length words the effect is
plain — the same draft laid out at 8px-apart widths differs by 2 to 5
lines, while at equal widths a textarea and a div agree exactly.

The three layers now reserve the gutter together, in the shared metrics
block that already exists to keep them symmetric. `overflow: hidden` is
still a scroll container, so the non-scrolling layers honour it: 8px is
reserved on each, measured. The cost is a text column 8px narrower on every
platform, which is the price of one geometry rather than a per-platform one.

The browser scenario asserts the premise directly — equal wrap widths, and a
reserved band greater than zero on each layer. The band is what stops the
assertion being vacuous: the widths would also match with no reservation at
all on this engine's overlay scrollbar, and it is the reservation, not the
match, that carries the guarantee to a platform whose scrollbar takes real
width. Removing the declaration fails it with `expected 0 to be greater
than 0`, and fails the golden with it.

Also from the same round, three comment corrections: the e2e file header no
longer describes the deleted layout effect, the measurement guard no longer
claims the backdrop holds exactly one text node (the sentinel makes a
second), and the sentinel comment now carries the one-sidedness argument
that also settles the ghost hint — the mirror only fails when the backdrop
is SHORTER, and the hint can only add content, never remove a line box.

apps/web/tests/snapshots/composer-draft-scroll/geometry.expected.md

@@ -5,6 +5,8 @@
 - draft overflows the capped box: true
 - visible lines: 14
 - both layers share one scroll extent: true
+- both layers wrap at one width: true
+- scrollbar gutter reserved on each layer: 8px / 8px
 - textarea scroll offset: 0px
 - glyph layer tracks it: true
 - first draft line is on screen: true