fix: graceful fallback for missing external MCCs

[midwan]

Summary

BetterString.mcc and HotkeyString.mcc were hard runtime dependencies of the preferences editor (MCP). If either was absent, MUI_NewObject returned NULL and the NULL was passed as a MUI Child tag, crashing the system.

Changes

• ostring() — try BetterStringObject first, fall back to built-in StringObject on failure
• CreatePrefsGroup() — pre-create the HotkeyString + snoop button HGroup outside the MUI tree; fall back to built-in Keyadjust when HotkeyString.mcc is not installed. Snoop notification setup is guarded by a NULL check.
• InputClass.cpp — guard SimpleButton(Value) against NULL Value on <input type="button"> without a VALUE attribute (matching the existing guard in the Input_Submit case)
• README — new README.md with runtime requirements table documenting all optional MCC dependencies and their fallback behavior, plus a changelog covering work since v12

Audit findings (no fix needed)

Component	Finding
InputClass.cpp / IsIndexClass.cpp — BetterStringObject	Already handles NULL via FLG_Layouted skip
TextAreaClass.cpp — TextEditorObject	Already handles NULL gracefully
SelectClass.cpp — CycleObject	Built-in MUI class, NULL-checked
ScrollGroup.cpp — ScrollbarObject / RectangleObject	Built-in MUI classes, always succeed
CreatePrefsGroup() — CacheSliderClass / GammaSliderClass	Internal classes, guaranteed created before use