Live-expose monitor split, clean up leftover debug diagnostics

srd.monitor.split only ever ran at Lua config load despite being a plain
WindowManager mutation that every backend's monitors() already reads
fresh on each call. Adds srd dispatch set output split <name|id> <parts>
[rows|columns] (IPC set_monitor_split), same id-resolves-to-name pattern
set_output_enabled already uses.

Also removes eight log::warn!("XXX-DIAG ...") lines left behind from live
debugging in the multi-session shift that landed in 3c41fc4 - the same
"temporary, never removed" pattern already fixed twice earlier this
session. Several fired on genuinely constant interaction (every title
change, every workspace switch, every layer-shell surface hide), not
just a one-off leftover. Left xdg_shell.rs's own POPUP-GEOM-DIAG/
POPUP-GRAB-DIAG alone - that one is a still-open, self-documented
investigation, not litter.

Also documents (docs/TODO.md, not a code change) a live incident where
creating a second fake monitor visibly corrupted the real monitor's
position and kept drifting with no further input - not root-caused
srdwm-side, flagged to the AGS peer session since a fake monitor's real
wl_output global is indistinguishable from a real hotplug to GDK/GTK.
And documents a deliberate decision not to blind-port window decoration
rendering onto the experimental, never-live-tested GPU render path.

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