feat(ios): LibIOS binding layer (226 funs) + push notifications landed

[zendrx]

What this PR does

1. LibIOS — the iOS binding layer (src/native/engine/ios/ios_bindings.cr) — the headline

The framework's iOS branches reference LibIOS.* 225 distinct functions across ~25 files — and none of them were ever written. This PR writes the complete lib LibIOS C-ABI contract:

• 226 funs, each signature derived from its actual framework call sites (not guessed): widget creators return Void* handles, string getters return borrowed UInt8* released via free_string / free_string_array / free, colors are normalized Float32 components, music time is Float64, sensor/battery/etc. per its usage.
• Mechanically audited: 225/225 referenced names bound, zero missing, and a call-site arity scan caught and fixed one real conflict (platform#share called share with 2 args while share.cr uses the 6-arg form — platform now passes NULL for absent fields).
• Documented ABI conventions at the top of the file (handles, string ownership, threading, main-thread callbacks) so the Swift/ObjC libnative_ios dylib can be implemented directly against it.
• Includes String#to_utf8 (the iOS branches' string convention) in the same file.

2. Push notifications (from the interrupted session)

• The never-compiled push_notification.c_r from the repo root moved to its rightful place src/native/framework/push_notifications.cr and fixed: null-checked method ids, class-ref cleanup, new handle_token_refresh/on_token_refresh, exception-safe user callbacks (an app exception can no longer unwind through the JNI boundary).
• Java bridge thread-safety: PushManager/FcmService now post every native callback to the main thread via Handler(Looper.getMainLooper()) — FCM/GMS executor threads were calling into the thread-bound cached JNIEnv (UB). Dead reflection listener removed; null tokens no longer cross JNI.
• Wired into src/native.cr + require "log"; desktop-path spec coverage added (spec/push_notification_spec.cr); Changelog has an Unreleased section.

3. Note on CI (owner is checking personally — probe step removed)

While auditing I found the platform typecheck jobs pass despite 239→225 LibIOS references existing with no definition — meaning -Dnative_ios/-Dnative_android are likely not activating the flag?(:…) branches and the jobs are desktop compiles in disguise. A temporary probe step was added to ci.yml and is removed in this PR per owner decision. Once this PR merges, a good manual check is: crystal eval -Dnative_ios 'puts {{ flag?(:native_ios) }}' — if that prints false, the -D handling in the installed toolchain needs a look before the iOS branches can be considered genuinely type-checked.

🤖 Co-authored with the repo owner's in-flight batch work.