fix(ime): implement set_preedit_region and fix done_serial tracking for Korean IME

[kkimdev]

This is PR 2-1 of a stacked series (split from #27 for reviewability). PR 1 (#28) was a behavior-neutral refactor.

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Last syllable lost when committing Korean text via Space/Enter

Composing "가나다" and pressing Space or Enter drops the final syllable "다".

Cause: Two bugs:

1. set_preedit_region was a no-op. ChromeOS IME calls this on every keystroke to describe where the composition starts relative to cursor. Without it the cursor offset is wrong when committing.
2. done serial was hardcoded to 0. Guests validate serials — they discard updates where the serial has not advanced.

Fix:

1. Translate set_preedit_region into v3 delete_surrounding_text + preedit_string + done.
2. Add with_state helper that manages a monotonically increasing done_serial; wire to all done-sending paths.

Known limitation: When the IME commit is followed by an Enter key event, the fix is incomplete. This requires the serial/activation changes from the next PR in the stack.

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 sommelier/src/handler/text_input.rs | 296 +++++++++++++++++++----
 sommelier/src/state.rs              |   2 +
 2 files changed, 259 insertions(+), 39 deletions(-)

[kkimdev]

Replaced by full core-2 PR