fix(wrap): keep wrapped text and cursor visible

[srothgan]

Problem

Active wrapping could place the native cursor outside the textarea when content exactly filled the drawable width. In word-based modes, oversized words could also be clipped, and a trailing whitespace run could move atomically to a continuation row, briefly placing the cursor in the middle of an otherwise blank row.

Change Summary

• Centralize text width, line-number width, and reserved-caret width in shared layout metrics used by measurement, rendering, cursor mapping, and hit testing.
• Reserve a visible caret cell in every active wrapping mode while preserving horizontal scrolling for WrapMode::None.
• Split oversized word content at grapheme boundaries so wrapped text is never clipped.
• Make Word and WordOrGlyph behavior equivalent while retaining WordOrGlyph for source and serialization compatibility.
• Consume whitespace runs one grapheme at a time so spaces fill the current visual row before excess whitespace wraps.

Test Evidence

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo check --offline
• cargo test
• Windows-supported backend and feature matrices for crossterm 0.28, termwiz, no-backend, search, serde, and arbitrary
• Full claude-code-rust consumer test and clippy suites against the local path dependency
• Manual reproduction with trailing spaces at the right wrapping boundary

Breaking Changes

No source-level breaking changes. Wrapped modes can wrap one cell earlier because they now reserve space for the native caret. Word now splits oversized words at grapheme boundaries instead of allowing trailing content to be clipped, and WordOrGlyph remains available with the same corrected behavior. WrapMode::None retains horizontal scrolling.