Extends the merged isearch (#70) with regex matching: both dedicated
entry keys (C-M-s / C-M-r) and a mid-search toggle (M-r), with
multi-line matches. Q#RX1–RX6 stances + bets.
Key survey finding recorded: the GPU's push_glyph_extent_rects already
fans a byte range across visual lines (multi-line selections use it),
so multi-line needs no GPU rendering change — only the TUI SearchView
(which assumed single-line) mirrors paint_local_selection's per-row
wash. SearchPrompt gains regex/invalid → protocol v10.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
