Fix workspace switches undoing themselves within milliseconds

sync()'s per-tick platform.focus(id) re-assertion (added to keep real
Wayland/X11 keyboard focus following core's own bookkeeping) ran
unconditionally on every dirty tick, including when nothing about focus
had actually changed. focus_window (core) has its own, separate side
effect of switching to the focused window's workspace when it differs
from the current one - correct when focus genuinely moves to a window
on another workspace, but this call was never gated on focus having
changed at all: switching workspace via activate_workspace left the
still-focused window's own workspace field untouched, so the very next
dirty tick's blind re-assertion of that same focus saw a mismatch against
the just-changed current_workspace and switched straight back.

Confirmed live via temporary core-side logging: two switch_workspace
calls a few milliseconds apart, the second one undoing the first every
single time, for every workspace switch that didn't also change which
window was focused.

Gated the re-assertion on the focused id actually changing since the
last sync() call. Real focus-follows-real-platform-focus still happens
on every genuine change, which is all the original fix needed.

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