fix(ime): forward real serial and time in on_keysym

[kkimdev]

Split from #30 for easier review.

wl_keyboard::key events forwarded from the IME's on_keysym handler used
serial=0 and time=0, which can cause the guest compositor to reject or
misorder them.

Fix

• Pass the real serial and time from on_keysym instead of hardcoded 0.
• Add store_host_serial helper and host_serial field to capture the IME
  serial for later use by commit_state (PR 7).

Changes

• on_keysym: forward serial, time to wl_keyboard::key (opcode 3)
• New: store_host_serial(ctx, host_id, serial)
• New: TextInputState.host_serial (initialised to 0)
• 3 tests

[InternetOfTofu]

Hi kkimdev,

I tried to see this PR in action but was unable to reproduce it. (RUST_LOG=trace and can't see on_keysym in log).

I tried "Chinese Pinyin" and "Japanese with US keyboard" IME, but did not see this event.

Can you share how to reproduce it? or if it is not possible to repro it on a US chromebook keyboard?

Thanks!

(I'm happy to merge it if it's not possible to repro with my devices. If it is possible I want to run it before merging it)