Fix/win32 ime setcontext lparam

[Touma-Kazusa2]

• ## Summary

Fix Win32 IME UI flag masking in the WM_IME_SETCONTEXT path.

Problem

For WM_IME_SETCONTEXT, IME UI visibility flags (including ISC_SHOWUICOMPOSITIONWINDOW) are carried in lParam.

The current code masks ISC_SHOWUICOMPOSITIONWINDOW on wParam, so the intended flag is not actually cleared.

Reproduction / Observed behavior (Windows + Microsoft Pinyin)

• On first IME input after app launch, an extra composition/preedit UI may appear.
• After window focus is lost and regained, the first IME input can reproduce the same issue.
• Once the current composition is committed (Enter/candidate selection) or fully canceled (delete all preedit text), the
  extra UI disappears.
• Subsequent IME input in the same focus cycle is typically normal.

Behavioral linkage (cause -> effect)

1. WM_IME_SETCONTEXT is the context-(re)activation point (startup first input / first input after refocus).
2. In this message, IME UI visibility control is carried by lParam (ISC_* flags).
3. winit intends to suppress the default IMM composition UI in this path before forwarding to DefWindowProcW.
4. Previously, the mask was applied to wParam, so the intended suppression did not actually affect the UI-flag carrier
   (lParam).
5. This can surface as an extra IME composition/preedit box when context is activated.
6. Commit/cancel ends the current composition session (e.g. WM_IME_ENDCOMPOSITION), so that extra box disappears with
   the ended session.
7. In the same focus cycle, WM_IME_SETCONTEXT is usually not re-entered, so the faulty path is not retriggered until the
   next context reactivation.

Change

Mask ISC_SHOWUICOMPOSITIONWINDOW on lParam (not wParam) before forwarding to DefWindowProcW.

Why changing lParam here is expected

This does not mutate the OS message source. Windows still provides the original wParam/lParam for WM_IME_SETCONTEXT.

Per Microsoft docs, apps that handle composition display should clear ISC_SHOWUICOMPOSITIONWINDOW in lParam before
calling DefWindowProc.

• Existing code already intends to hide IME-drawn composition UI in WM_IME_SETCONTEXT.
• The issue is operand mismatch: ISC_SHOWUICOMPOSITIONWINDOW is a lParam flag, but the mask was applied to wParam.
• This PR keeps the same intent and fixes only the target operand (wParam -> lParam) before forwarding to DefWindowProcW.
• So this is a correctness bugfix, not a behavior-policy change.

References

• WM_IME_SETCONTEXT
  https://learn.microsoft.com/en-us/windows/win32/intl/wm-ime-setcontext

  “the application must clear the ISC_SHOWUICOMPOSITIONWINDOW value from the lParam parameter”

• WM_IME_ENDCOMPOSITION
  https://learn.microsoft.com/en-us/windows/win32/intl/wm-ime-endcomposition

  “This message has no parameters.”

• IMR_COMPOSITIONWINDOW
  https://learn.microsoft.com/en-us/windows/win32/intl/imr-compositionwindow

  “can be sent by the IME to a window that has cleared the ISC_SHOWUICOMPOSITIONWINDOW flag”

Scope

Windows backend only (winit-win32).

Validation

• cargo test -p winit-win32
• Downstream manual verification (Windows):
  • Reproduced with bevy 0.18 example: examples/input/text_input.rs (Microsoft Pinyin).
  • Before patch: behavior matched the reproduction notes above.
  • After patch: the above behavior is no longer reproduced.

Related

• Related to Composing text display when input Chinese #3893 (same extra IME composition box symptom, different trigger conditions).

Checklist

• Tested on all platforms changed (Windows)
• Added an entry to changelog for this user-visible change
• Updated documentation (N/A: no API/documentation surface change)
• Added/updated example (N/A: bugfix in existing Win32 path)