diff: the dialog opens every file it lists, and says nothing when closed - #98555

Summary

TLDR: in the /diff dialog every listed file opens its diff; closing the dialog prints nothing

Notes

Without the fullscreen layout /diff opens a dialog that lists every changed file. A file edited before the session began, or a test or generated file such as a lockfile, stayed on "Loading diff…" when opened there. Closing the dialog with Esc printed "Diff dialog dismissed" under the plugin's name.

• The dialog lists every changed file, but hunks were read only for the rows the docked pane draws: the session's files, and the earlier ones once their section is opened. The dialog has no such section to open, so those rows were never read.
• Hunks are now read for the rows the pane's seat lists. Docked, nothing changes. In the dialog that is the five rows in view, read again as the selection moves the view; a row already read, or being read, is not asked for again.
• Walking a dialog of twelve files from the first row to the last starts git eight times for hunks: once for the first five rows, once for each row that comes into view. No file is read twice.
• A person closing the dialog prints nothing. /diff typed while the dialog is open still answers "Diff dialog dismissed".
• The oldFiles test fixture takes the file names it answers for; its three existing callers are unchanged.

Test Plan

• claude plugin test mods/diff passes: 210 pass, 0 fail
• Without the fullscreen layout, in a repository with one file edited before the session and one after: /diff, move to the earlier file, Enter: its hunk shows
• The same with a lockfile among the changes: its hunk shows
• Twelve changed files, the selection walked from the first row to the last, Enter and Esc on the first, sixth and last: each shows its hunk; eight git processes for hunks, no file asked twice
• Esc on the dialog prints nothing; /diff typed over it answers "Diff dialog dismissed"
• With the fullscreen layout: the pane docks, a session file shows its hunk, the earlier files show theirs once "(show)" is pressed
• A git that fails for one row: that row reads "Diff unavailable", the next row still opens

Revert-proof: 3 new tests fail with the two modules' changes reverted (the mod's own suite, run against the unchanged modules)

Changelog