Split ipc.rs into ipc/ by concern, and fix a stale README

Codebase modularization, requested directly. Surveyed the whole
workspace first: at ~38k lines it's already organized by topic
(crates/core/src/manager/, crates/wayland/src/{state,udev,decoration}/
already split into small per-concern files) - crates/platform/src/
ipc.rs was the one real outlier, 1894 lines holding the socket
lifecycle, every payload type, both dispatch match statements, and its
own tests all in one file.

Split into ipc/{mod,types,dispatch,tests}.rs by concern, matching the
established pattern exactly - mod.rs keeps IpcServer itself, types.rs
the response/event structs and snapshot functions, dispatch.rs
handle_request/handle_set, tests.rs the existing suite moved verbatim.
Extracted via exact line-range copies against git's own HEAD content
(not retyped), specifically to rule out a transcription bug in a file
this central. Pure reorganization: build/test/clippy clean before and
after, exact same test count (29 in crates/platform) both times.

README.md separately corrected: it still linked to legacy-cpp/ (deleted
this shift) and described the Wayland backend as the smaller, less-done
one - backwards from current reality, where Wayland is the daily-driver
target and by far the more complete backend.

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