Set commands apart with a line and a shade - #23

Closes #13. The terminal draws a thin line above each command's prompt and shades every other command, prompt and output together. A command that failed gets a bar beside it in the margin. Lines and shades are on by default, each with a switch in Settings › Appearance › Commands (zsh, with shell integration).

Screenshots

A line above each prompt, every other command shaded, a red bar beside the failed make lint, and the shade of the prompt you're at fading out below the cursor.

The four themes: the line, the shade and the bar all come from the theme's colors.

A running command has the bar in the accent color.

Summary

 <PaneView>
+  <CommandSeparatorView>     # under the terminal: shades, lines, margin bars
   <CodecTerminalView>        # background already clear, so text keeps its contrast
   <FindHighlightView> …

refreshCommandSeparators          (scroll, output tick, prompt drawn, command started, layout, settings)
  off, no integration, or full-screen program  → draw nothing
  rows moved by a resize                       → draw nothing, find the commands again 0.3 s after the last step
  CommandBlocks.spans(overlapping: screen)     → each command's rows, every other one marked
  per command:
    line at its top (through the middle of a prompt's blank first line)
    shade from its line to the next; the newest fades out below its last row
    bar in the margin if it failed (alert) or is running (accent); nothing if it worked

• The line spans the text, not the pane, so it doesn't read as a split. The shade spans the pane.
• The bar sits where the mark over a command gone back to is drawn, a little quieter. It follows the "Line between commands" switch.
• The prompt waiting for a command counts as the next command. Pane now remembers where zsh marked it (133;B), because between a command finishing and its prompt the cursor alone doesn't say where the output ends.
• At each prompt, CommandBlocks.forgetBlocksGone drops the commands whose command line is no longer on screen. After clear or ⌃L they used to linger until the next command ran.
• The refresh runs at every frame of a scroll, so it copies the screen once instead of going through TerminalGrid: about 5.6 ms per refresh in a debug build on a 297×86 terminal, down from about 32 ms. Not measured in a release build.

Evidence

• Before:
  
  After: the first screenshot above.
• New tests, all passing with the rest of the suite (566 tests) and the universal release build:
  • CommandSeparatorTests (real zsh panes): a line above each command and a shade on every other one, each setting on its own, a pane leaving and rejoining its window, a prompt with a blank first line, bars beside a failed and a running command, clear, a full-screen program, a resize that rewraps.
  • CommandBlocksTests: spans, commands past the cursor left out, every other command staying the same one as old ones are dropped, forgetting cleared commands.

Merge Danger

Door: two-way

Two new preferences default to on. Turning both off, or reverting, restores the old look; nothing stored changes shape.

Blast Radius: terminal

• Every zsh pane looks different on first launch after updating, which is the point of the issue but will surprise people who didn't ask for it.
• promptWasDrawn now prunes blocks when a command line is gone from the screen. Copy Output and Select Output read blocks, so a wrong prune would disable them for that command until the next relocation. Timeline entries are untouched.
• A per-frame screen copy during scrolling in integrated panes with either setting on.
• Pressing Return at an empty prompt moves the line to the newest prompt; the empty ones above get none.
• All 13 images in docs/images were retaken. The hero's git log shows this branch's name, because the tour clones the repository.

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