diff: add a true in-place "inline" overlay (virtual text) as the future layout = "inline"

まず現在の layout = "unified" の経路を追跡し、#293、#195、#169、#277、#205 で参照されているネイティブの diff 動作と比較します。単一バッファの extmark ビュー、編集可能な追加行、仮想的な削除行、承認/拒否の動作、および ClaudeCodeDiffOpened / ClaudeCodeDiffClosed autocmd の処理を定義します。完了の条件は、提案する inline モードが追加のウィンドウなしで動作し、レビュー時の編集を保持することです。

Summary

Follow-up to #293 / #195. The diff_opts.layout = "unified" option (renamed from "inline" before v0.4.0) renders a unified diff in a separate vsplit pane — a single read-only buffer with interleaved red/green lines. This issue tracks adding a true in-place inline overlay: the diff shown as virtual text layered onto the file in the same window, the way users typically picture a "VS Code-style inline" diff (cf. mini.diff, gitsigns preview, sidekick NES, Cursor inline edits).

When implemented, this mode is the one that should claim layout = "inline" — which is exactly why we reserved that name in #293.

"unified" vs the proposed "inline"

• "unified" (shipped): single read-only buffer in a rightbelow vsplit, deleted (red/strikethrough) + added (green) lines interleaved. Compact, but it's a separate pane and not editable before accept.
• "inline" (this issue): no extra window — proposed changes rendered in place over the real file.

Suggested approach (keeps pre-accept editability)

The unified/read-only path forfeits editing the proposal before accepting. A prototype pattern that keeps it (the codecompanion/sidekick model):

• Proposed/added lines = real, editable buffer lines, highlighted (e.g. ClaudeCodeInlineDiffAdd) with a + sign.
• Deleted lines = virtual text via nvim_buf_set_extmark virt_lines (strikethrough/red), so they're visible but not part of the editable content.
• On accept, the buffer content already is the new file (deleted lines were never real text), so edits made during review are captured. On reject, clear the extmarks / restore.

A pure-Lua, zero-dependency engine (not mini.diff/diffview — see #169) keeps this aligned with the project's philosophy. Bonus: a single-buffer extmark view sets no &diff windows, which sidesteps the closeAllDiffTabs foreign-diff class (#277) and the stale-diff repaint issues (#205).

Notes

• The current "unified" path doesn't fire the ClaudeCodeDiffOpened / ClaudeCodeDiffClosed User autocmds that the native diff does — worth wiring up for any new mode.
• @wookayin expressed interest ("once we implement that") on #195 — collaboration welcome.

Refs #293, #195, #82

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