Phase B: main-thread sleep parks its context; the real D blocker is DOM entries

wasm/shims/context_sleep.cpp: a main-thread nanosleep whose frame stands on a
scheduler context that OWNS the stack arms a mailbox wake and yield_parks that
context instead of suspending the stack in place. It lives in the sleep
primitive rather than in tool_manager.cpp on purpose - KiCad and the wx core
stay untouched (CLAUDE.md's fork rule) and the whole K7 class moves at once,
not just TOOL_MANAGER::RunSynchronousAction's spin loop.

MEASURED AT D-ON, and it is NOT what unblocks Phase D. The four canvas-tool
specs still fail, but the trace now names a different cause: the fatal swap is
old=<libcontext ROOT> new=<tool coroutine> with mouseEventHandlerFunc above it
- a DOM mouse handler entering wasm DIRECTLY on the main stack, bypassing the
tick. So one coroutine is entered two ways: by the tick through the dispatch
context as a STAR TRANSFER, and by DOM handlers as a DIRECT SYMMETRIC SWAP. A
capture written by one path cannot be rewound by the other -> index out of
bounds in doRewind. That is section 7 rule 5 (partial migration is worse than
none) in its purest measured form, and it is why the harness stays green: its
coroutines are only ever entered from one place.

So the next increment is the DOM event entries (mouse/key/wheel/resize must
hand their events to the dispatch context as the tick does), not another park
site. It subsumes the one-root work too: with no dispatch on the main stack,
resolve_root_identity() always answers "the running context".

Landing state: STAR_DISPATCH=0, kicad 139 passed / 1 (pre-existing occ-probe)
= baseline, with the sleep shim in and inert.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LBjomQfKyRa3jBdeAKpmTw

scripts/kicad/build-kicad-target.sh

@@ -506,7 +506,14 @@ if [ "${APP_NAME}" = "kicad_tools" ] || [ "${APP_NAME}" = "occ_service" ]; then
     NANOSLEEP_YIELD_LINK=""
 else
     emcc -c -pthread "${PROJECT_ROOT}/wasm/shims/nanosleep_yield.c" -o "${STUBS_BUILD}/nanosleep_yield.o"
-    NANOSLEEP_YIELD_LINK="${STUBS_BUILD}/nanosleep_yield.o"
+    # Its scheduler-aware half (docs/features/async/22 Phase B): a main-thread
+    # sleep on a scheduler context parks THAT CONTEXT instead of suspending the
+    # stack in place. C++ because the registry is a header-only C++ layer in
+    # wx's port, hence WX_CXXFLAGS for the include path (the same reason
+    # thirdparty/libcontext needed it at Phase A).
+    em++ -c -std=c++17 -pthread ${WX_CXXFLAGS} \
+        "${PROJECT_ROOT}/wasm/shims/context_sleep.cpp" -o "${STUBS_BUILD}/context_sleep.o"
+    NANOSLEEP_YIELD_LINK="${STUBS_BUILD}/nanosleep_yield.o ${STUBS_BUILD}/context_sleep.o"
 fi
 
 # mallinfo() stub for the mimalloc build: -sMALLOC=mimalloc doesn't export the