A shell terminal could render output diagonally (each line shifted one
column right) after a full page reload or a cursor-query-failure replay.
Root cause: capturePaneBuffer's full-history path (capture-pane -p -e -S -)
and its cursor-query-failure fallback returned raw scrollback, which tmux
joins with a BARE \n. The browser xterm uses convertEol:false (correct for
the live PTY stream, which carries real \r\n), so each bare \n dropped a
row without returning the cursor to column 0 -> staircase. The visible /
tab-switch path (formatPaneSnapshot) was immune because it repaints each
row with an absolute cursor CSI.
Fix: new pure helper normalizeScrollbackEol() (\r?\n -> \r\n, idempotent
on CRLF, leaves lone \r overwrites untouched, adds/removes no rows) applied
at both raw-return seams. The absolute-positioned snapshot path is unchanged.
Tests: test/tmux-scrollback-eol.test.ts pins the invariant (no LF without a
preceding CR) + CRLF idempotency + lone-CR preservation. 136/136 across
tmux-scrollback-eol + tmux-capture-full-history + tmux-manager +
routes/session-routes; build, tsc, prettier, frontend-syntax clean.
