fix(ios): restore backend ABI parity, and gate it in CI

[nicolas-maman]

What I found

While checking whether #22's UIKit backend was actually complete, I compared the
ABI in aether_ui_backend.h against what each backend defines. Seven entry
points were on AppKit, GTK4 and Win32 but not on UIKit:

aether_ui_app_quit_impl
aether_ui_set_width_impl / aether_ui_set_height_impl
aether_ui_get_width_impl / aether_ui_get_height_impl
aether_ui_canvas_draw_image_borrowed_impl
aether_ui_canvas_draw_image_scaled_borrowed_impl
aether_ui_modifiers_impl

An iOS app calling any of them fails at link.

Why nothing caught it

This is the part worth fixing properly. Phase 1e compiles the UIKit backend and
links it against tests/ios/link_stub.c, which proves that everything it
references resolves. It says nothing about what it omits. The Win32 lane
only cross-compiles. So a missing entry point is invisible to every phase in
CI, because nothing in CI calls it.

The fix

All seven implemented. Two are documented no-ops with a stated reason, which
is different from a gap:

• app_quit does nothing on iOS. Terminating your own app is grounds for
  App Store rejection; the platform expects the user to leave via the home
  gesture. A cross-platform app with a Quit item now links and no-ops rather
  than failing to build for iOS.
• modifiers returns 0. A touch carries no modifiers, and UIKit has no
  pollable global modifier state. A hardware keyboard's modifiers arrive
  attached to the key event (UIKey.modifierFlags), which is a different
  question from "what is held right now".

get_width / get_height round the frame's edges rather than its size, so
iOS carries the same tiling guarantee the other three backends got in #101.

The gate

tests/scripts/check_backend_parity.py, wired in as phase 1c2: every function
declared in aether_ui_backend.h must be defined either in the shared sources
(aether_ui_system_extras.c and friends, which every backend links) or on all
four backends.

It distinguishes a definition from a declaration by scanning from the name to
whichever of ; or { comes first, rather than with one regex. That matters:
my first attempt used a single pattern and reported Win32 as missing
notify_impl, which Win32 defines at line 9779. A non-greedy match starting
earlier had swallowed it. A checker that under-reports is worse than none.

Verification

• Parity check passes: every ABI entry point on all four backends.
• Verified it fails correctly: removing the four size functions again makes
  it report all four and exit 1.
• aeb .all.ae builds clean on AppKit, zero warnings.
• The UIKit file itself cannot be compiled here (no iOS SDK on this machine,
  only CommandLineTools), so phase 1e's -Wall -Werror simulator build is the
  compile check. The additions mirror the AppKit implementations and the file's
  existing idioms (static const char association keys, objc_setAssociatedObject).

On #22

With this merged, every item in #22's work list is implemented and the ABI is
complete on UIKit. Its stated prerequisite, aether-lang-dev/aether#1385 (the
iOS --emit=lib target), is still open and is the remaining blocker for
actually shipping an app, so I have left #22 open rather than closing it from
here.

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