fix: correct the pi wheel docs and comments (release review)

The gate comment in terminal-ui.js said a stale-false flag falls through
to the PageUp/PageDown fallback, which is false for pi since the second
commit of this PR removed that fallback. The FOOTGUN comment above it made
the same claim. Say what happens instead.

"Every pi below 1.0.0 emits no mouse enables" was too absolute: regular
was the default there, but fullscreen exists from 0.84.0. Measured with
--tui-mode fullscreen under tmux on 0.84.1, 0.85.0, 0.86.0 and 0.99.2:
alternate_on=1, mouse_any_flag=1, mouse_sgr_flag=1. Default settings on
0.99.2 report all 0.

Document that Wheel Scrolls Local History leaves the wheel dead in
fullscreen pi (wiki, integration guide, changeset), add a pi block to
Troubleshooting, cross-reference the accepted exception from the dead
gesture rule in architecture-invariants, and mark the pi-integration-plan
passages this PR reversed as superseded.

Comments and docs only, no runtime change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changeset/pi-wheel-replay.md

@@ -2,4 +2,4 @@
 "aicodeman": patch
 ---
 
-pi: the mouse wheel no longer goes dead after a tab switch or reload. pi's fullscreen TUI, the default since 1.0.0, enables mouse tracking once at start, and Codeman's replay dropped that mode in the browser, so the wheel landed in an empty local buffer. pi now takes the same path as Copilot: the server strips the mouse enables and remembers them, and the wheel is forwarded from that record, on the primary pane, tiles, split Pane B and phones. A drag in fullscreen pi now selects text in the browser instead of using pi's own selection. A pi in `tuiMode: regular` is unaffected. Thanks to @Randalix for the report and the fix.
+pi: the mouse wheel no longer goes dead after a tab switch or reload. pi's fullscreen TUI, the default since 1.0.0, enables mouse tracking once at start, and Codeman's replay dropped that mode in the browser, so the wheel landed in an empty local buffer. pi now takes the same path as Copilot: the server strips the mouse enables and remembers them, and the wheel is forwarded from that record, on the primary pane, tiles, split Pane B and phones. A drag in fullscreen pi now selects text in the browser instead of using pi's own selection. With Wheel Scrolls Local History on, the wheel does nothing in fullscreen pi: the setting pins it to a local buffer pi leaves empty, and pi has no paging fallback. A pi in `tuiMode: regular` is unaffected. Thanks to @Randalix for the report and the fix.