[ryutaro-asada]

Closes #10. Stacked on #15 (based on its branch because both change highlight_line); merge #15 first.

Summary

scrollbind/cursorbind only mirror scroll offsets, so with insertions/deletions the two sides drifted apart. With the new diff.align (default true), the scratch buffers are rewritten with real filler rows, so line N on the left always corresponds to line N on the right. Scrolling and cursor movement stay aligned, like diff mode.

 4    { id: 2,  name: "Americano",  price: 350 },   │ 4    { id: 2,  name: "Americano",  price: 350 },
    ---------------------------------------------   │ 5    { id: 6,  name: "Macchiato",  price: 400 },  ↑ moved from line 8
    ---------------------------------------------   │ 6    { id: 7,  name: "Flat White", price: 440 },
 5    { id: 3,  name: "Cappuccino", price: 420 },   │ 9    { id: 3,  name: "Cappuccino", price: 420 },
 ...
 8    { id: 6,  name: "Macchiato",  price: 400 },  ↓│    ---------------------------------------------

(excerpt of a headless screen dump of test/demo.rb)

• M.align_lines() is a pure function that builds the aligned contents and the original-line → buffer-row maps from vscode-diff's change blocks. The shorter side of a block is padded at the end of the block.
• All line/char/move highlights are still computed on original line numbers and placed through the row maps. Move markers keep original line numbers.
• Filler rows use a new PartialDiffFiller group (links to DiffDelete, configurable as highlights.filler) and the 'fillchars' diff character, fixed to the window so it doesn't scroll horizontally.
• Windows that show line numbers get a statuscolumn with original line numbers (blank on filler rows). It is reset if the view closes as the last tab.
• diff.align = false keeps the buffers verbatim (old behavior).
• The built-in diff fallback is unchanged (diff mode already aligns).

Approach as discussed in #10. Rejected: diff mode for alignment (its own diff can disagree with vscode-diff) and virt_lines filler (scrollbind doesn't count virtual lines).

Testing

• make test: 55/55 pass locally (Neovim 0.11.1).
• New spec/align_spec.lua (14 cases):
  • align_lines on insertions, deletions, uneven modified blocks, and multiple blocks.
  • Equal row counts; unchanged lines on the same row on both sides; filler highlight.
  • Highlights land on the shifted rows.
  • Original line labels; statuscolumn set only where numbers are shown.
  • Move markers keep original line numbers and sit on the right row.
  • align = false keeps the buffers verbatim.
  • A screen test that redraws and reads the number column.
• That screen test caught a bug during development: the statuscolumn callback assumed g:statusline_winid, which isn't set inside %{} items. It errored on every redraw. Fixed, and the test fails without the fix.
• Existing move tests now look rows up through the alignment map.
• Manually checked a headless screen dump of test/demo.rb with number on: filler rows, original line numbers, cursor stays on the same row after 6j in one window.
• CI (0.10.4 / stable): see checks.

Behavior change

Alignment is on by default. Copying text out of the diff buffers now includes empty filler rows; set diff.align = false to get verbatim copies.