docs: add backlog item for stale delete-only hunk-nav comments

moveToNextHunk and moveToPrevHunk both label their firstVisibleInHunk < 0
guard as skipping delete-only hunks in collapsed mode. A delete-only hunk
keeps its first removed line visible as the placeholder, so that branch looks
unreachable and both comments name the wrong reason.

Surfaced by the review of #329, whose new positionOnFirstChange godoc states
the opposite and correct behavior.

Author: umputun

docs/backlog/hunk-nav-comments-claim-delete-only-skip.md

@@ -0,0 +1,23 @@
+---
+worth: later
+where: app/ui/diffnav.go:583
+added: 2026-08-20
+---
+# hunk-nav comments claim delete-only hunks are skipped, but they are not
+
+`moveToNextHunk` (`app/ui/diffnav.go:583`) and `moveToPrevHunk` (`:599`) both guard
+`firstVisibleInHunk(...) < 0` with `// skip delete-only hunks in collapsed mode`. That is not what
+happens. `isCollapsedHidden` (`app/ui/collapsed.go:415`) deliberately keeps a delete-only hunk's first
+removed line visible as the `⋯ N lines deleted` placeholder, so `firstVisibleInHunk` returns
+`hunkStart` for such a hunk and `]` lands on it. A mixed hunk always has a visible add, so it returns
+that. The `< 0` branch looks unreachable for any real hunk, which means both comments describe a case
+that does not occur and name the wrong reason for a guard that does.
+
+The godoc added for `positionOnFirstChange` (`:906`) states the opposite and correct behavior, so the
+package now contradicts itself in three places. Surfaced by the revmux review of PR #329 and confirmed
+by codex, which also pinned the real behavior in `TestModel_CollapsedHunkNavigationDeleteOnly`.
+
+Deferred rather than fixed inline because #329 was scoped to the `--start-at-change` flag and these two
+lines predate it. The fix is either correcting both comments to say what the guard is for, or
+establishing whether `< 0` is genuinely unreachable and dropping the branch. Worth deciding which
+before editing, since a comment that keeps a dead branch alive is the weaker of the two outcomes.