docs: the compositor list was six claims and one of them false

Mainline dwl has no input method: dwl.c creates a virtual keyboard manager
and never a wlr_input_method_manager_v2, so IME there is a third-party
patch and strans falls back to XIM through Xwayland -- not what the line
promised.  Being a wlroots or smithay compositor is the wrong criterion,
which is the general lesson: the compositor has to wire the protocol up.
river and labwc do, and were checked; Wayfire, niri and COSMIC never were.

So the opening says the criterion instead of a list to maintain, and the
Run section already says how the choice is made.  The README says it once.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -2,9 +2,9 @@
 
 strans is a small, single-user input method for Korean, Japanese, English,
 emoji and symbols, with Vietnamese Telex as a compatibility mode. One
-engine serves four frontends: `zwp_input_method_v2` on Wayland — the
-wlroots and smithay compositors, so sway, river, labwc, Wayfire, dwl, niri
-and COSMIC — IBus, which GTK 4 and Qt use, XIM, and a GTK 3 module.
+engine serves four frontends: `zwp_input_method_v2` on Wayland — sway and
+any other compositor that offers it — IBus, which GTK 4 and Qt use, XIM,
+and a GTK 3 module.
 
 ## Modes