Editor: overwrite / insert-replace mode (Insert key)

[Copilot]

Editor is insert-only — no overwrite state, command, or caret style. TextView supports it; it's a widely-expected editor mode and not replicable by a consumer.

Core

• OverwriteMode property (field-keyword pattern, default off) + OverwriteModeChanged event on Editor
• Command.ToggleOverwrite / EnableOverwrite / DisableOverwrite wired via AddCommand; Insert key bound in DefaultKeyBindings (user-overridable [ConfigurationProperty])
• OverwriteAtOffset() helper: replaces the grapheme cluster at caret using RemoveAndInsert offset mapping so the caret anchor advances correctly. Wide-rune safe via StringInfo.GetNextTextElementLength. Falls back to insert at line-end (never consumes newline). Selection still replaces normally.
• Multi-caret insert path routes through the same overwrite logic per-caret
• Cursor forced to CursorStyle.SteadyBlock while overwrite is active

ted

• INS/OVR status-bar indicator, updated on OverwriteModeChanged

Specs

• specs/overwrite-mode/spec.md created
• specs/public-api.md updated with new property and event

Tests

• 11 new integration tests covering: toggle via Insert key, overwrite replacement, line-end insert fallback, selection replacement, single undo step, multi-line newline preservation, Enable/Disable commands

editor.OverwriteMode = true;
// Caret at offset 0, document "abc"
// Type 'x' → document becomes "xbc", caret at 1

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