state_exec: always built — remove CVC_STATE_EXEC (unbreaks gh-pages)

[transfix]

Problem

deploy gh-pages → Build the non-pthread demos has failed on every master push since c42e945:

src/cvc/ariadne/ariadne.cpp:630:20: error: no type named 'channel_policy' in namespace 'cvc::state_exec'

The gh-pages wasm build forced -DCVC_STATE_EXEC=OFF. That was the only build using the OFF configuration, so any unguarded state_exec type slipped through PR CI and only broke after merge (here, channel_policy_ from #467). State_exec is what Ariadne apps run on, and the demos are moving to .ari apps, so having an OFF configuration at all works against that.

Change: state_exec is always built; the CVC_STATE_EXEC option is gone

• CMake: removed option(CVC_STATE_EXEC), the core/state_exec/ source filter, the PUBLIC CVC_STATE_EXEC compile definition, and the five if(CVC_STATE_EXEC) test-gating blocks.
• C++: removed every #ifdef/#ifndef CVC_STATE_EXEC, keeping the ON code. The files are the Ariadne runtime, loader, app_runtime and net_intrinsics, core async_task, and pycvc Exec. The OFF fallbacks are gone: reactive slots that warned and still showed, run_init's error, the throwing Exec stubs, and the no-op residents and verbs.
• API: removed have_state_exec() from cvc::ariadne, AppRuntime and cvc::gl::AriRuntime, since it would now always return true. Its callers are gone too: nav_compute's runtime check, net_intrinsics' early return, and 79 C++ plus 9 Python test skip guards. Nothing in the sibling repos calls it.
• Builds: no wasm build forces OFF any more. The -DCVC_STATE_EXEC=… flags are gone from build-wasm-demo.sh (gh-pages), the cvcgl-examples and pycvc-gl-cp312 build-wasm.{sh,ps1} scripts, and publish-cvcgl-wasm.yml. cvcgl-examples cvc_revision goes 2 → 3 so its wasm-mt bundle republishes over 3.4.0+cvc.2.
• Docs: ARIADNE.md and the roadmaps are updated; historical checklist entries are annotated rather than rewritten.
• Side effect: a stale build dir with CVC_STATE_EXEC=OFF cached now just builds state_exec. That also fixes the native OFF executable-link failures (app::exec_scheduler(), nav_compute).

The commits are kept separate for review: the original channel_policy_ guard, then the wasm flag flip, then the option removal (which supersedes the guard).

Safe on non-pthread wasm

• Ariadne uses the stackless evaluator and async_scheduler, and neither creates threads. The only std::thread in state_exec is the recursive evaluator with a timeout, which nothing on the wasm paths calls.
• app.computePool() gets 0 workers without pthreads. emsdk's stub emscripten_num_logical_cores() returns 1, and I confirmed hardware_concurrency()==1 under node. It never calls pthread_create, which fails with ENOTSUP there. nav_city_swarm/nav_city_drive already rely on this in the shipped gallery.
• None of the nine current gh-pages demos use Ariadne yet. Once .ari demos use async verbs (compute_async, which does spawn a thread), they need the wasm-mt gallery or an inline fallback.

Verification

• gh-pages non-pthread build, locally (emsdk 5.0.7, against a threaded wasm deps prefix like CI's deps-wasm-mt): build-wasm-demo.sh with state_exec on finishes with rc=0. All 9 demos link and the gallery assembles. I served it and ran nav_city_swarm, the demo that uses computePool(): it runs (800 agents, render + minimap), and the only console error is the expected no-trained-weights fallback. After the removal commit, libcvc.a and libcvcGL.a rebuild cleanly under em++ (195/195).
• Native, minimal flags (no yaml): the full test tree builds (448/448) and ctest passes 3150/3151. The one failure, VolumeOpsTest.BackProjectDimensions, is because the minimal config sets CVC_FFT_PROVIDER=none, which this change doesn't touch.
• Native, stale CVC_STATE_EXEC=OFF cache, cvcGL + examples: all 297 steps build and link, including nav_compute and the executables that used to fail.
• Native with yaml-cpp + pycvc/pycvc_gl, built with the cvcpkg swig 4.4.1 (SWIG_LIB/SWIG_DIR from the cvcpkg prefix; both modules carry swig_runtime_data5): the full tree builds (659/659). All 290 Ariadne tests pass, including the yaml-gated program-lane ones that skip without yaml, and so do all state_exec suites and 26 pycvc tests, among them pycvc_exec, pycvc_exec_async, pycvc_exec_coro and pycvc_integration.
  • pycvc_ari, pycvc_gl_world and pycvc_gl_chase_parity fail, but unmodified origin/master (d6ac6a7) built the same way fails all three identically, so they are pre-existing and not caused by this PR. CI doesn't build pycvc, which is why nobody noticed. The errors: an if args: keepalive %pythonappend hitting a named-parameter ImGuiOverlay.__init__ (NameError: args), a StageLighting.get_stage() value mismatch, and pycvc_gl.camera not importable from the build tree.
• Dry-run dispatches (build + pack, no publish) on 71d8176, the flag-flip commit: publish-cvcgl-wasm (run 36660870376) and publish-pycvc-gl-wasm (run 36660872184).
• git clang-format origin/master: clean. clang-format re-sorted include blocks that merged once the #ifdef separators were removed.

deploy-pages only runs on push to master, so the real non-pthread step runs once this merges.