Surface next-word predictions after a commit

[mkpoli]

Stacked on #43 (targets feat/next-word-prediction-core; retarget to master once #43 merges).

Why

#43 added the prediction logic but nothing consumed it. This wires it into the live IME so the engine keeps working after a word is committed — the actual fix for "it only completes one word." Previously, once you committed a word the suggestion engine went dormant (key_event_sink.rs gated everything on a non-empty buffer) until you typed the next partial word.

What

After each commit (Kana mode, context_aware on), a popup shows the likely next words — no active composition. Per the chosen UX, acceptance is number-key only so normal typing is 100% untouched:

• 1-9 → insert that prediction as finalized text, then chain into the prediction for the following word (the sentence-building feel);
• Esc → dismiss;
• Space / Enter / any other key → passes through unchanged and drops the popup (a space stays a space).

Implementation notes:

• predicting state flag distinguishes the predictive popup from the completion popup.
• The popup anchors to the focus caret via the existing GetGUIThreadInfo path (candidate_window.rs), so it positions correctly with no composition.
• insert_committed mirrors commit but opens its own short-lived composition (there is none while predicting).
• context_aware is the natural off-switch — no new config surface.

Verification

• ✅ Builds and links the cdylib for x86_64-pc-windows-msvc (cargo xwin build --release); exports intact.
• ✅ cargo clippy -D warnings clean on the MSVC target.
• ⚠️ Behavioral verification on Windows is pending — TSF interaction (the popup, accept-and-insert, key gating) can't be exercised from Linux. Needs a Windows eyeball before we claim it works, same as the settings dialog.

To verify on Windows

1. Switch to Kana mode, type + commit a word (Space).
2. A numbered popup of likely next words should appear at the caret.
3. Press a number → it inserts that word and immediately offers the next prediction.
4. Confirm Space/Enter/typing behave exactly as before (popup just disappears).
5. Toggle suggestions.context_aware = false → popup should not appear.

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