Lag when opening or exiting programs: Primary Device Attributes query is never answered #21

Starting or exiting programs inside a dvtm pane can lag noticeably.

Cause
dvtm never replies to the Primary Device Attributes query (ESC [ c / ESC [ 0 c). The c verb isn't handled in interpret_csi() in vt.c. Many programs send DA1 at startup/exit, often as a sentinel after other queries such as OSC 11 background color or CSI ? u, and then block until a reply comes or a timeout expires. Examples: fish 4, neovim, helix, yazi, and bat/delta (terminal-colorsaurus). A real terminal replies right away, so the same programs start instantly outside dvtm.

Reproduction (query from inside a pane, 2s timeout)

DA1: NO REPLY (timeout 2s)
DSR: b'\x1b[1;0R' in 0.4 ms

Ruled out

• pane create/destroy path (fork, buffer alloc/free, arrange/redraw): ~3–6 ms per create/exit, same as upstream 7bcf43f
• no regression from the recent color-pair changes

Fix
Reply to DA1 with ESC [ ? 6 c (VT102, same as st). Leave DA2/DA3 (CSI > c, CSI = c) unanswered so they don't get a DA1-shaped reply.

Side note (not fixed here)
The cursor position report (CSI 6 n) replies with 0-based row/column. It should be 1-based.