W7e: delete the native unit pipeline (.so compile + dlopen execution)

Units now run exclusively on wasm; the native generated-C++ -> clang -> .so ->
dlopen path and its request-time fallback are removed.

- Dispatch (linux_fastcgi.cpp): the 4 handle_complete branches + the CLI-socket
  path route every request through wasm (wasm_ready compiles cold/stale on
  demand); a wasm-unavailable unit now yields a clean error page
  (fail_wasm_unavailable / render_request_failure) instead of native execution.
  compiler_invoke / _cli / _websocket / _serve_http deleted.
- compiler.cpp (-1274): removed the native .so compile (COMPILE_SCRIPT),
  load_shared_unit, dlopen/dlsym/dlclose, compiler_load_shared_unit, and the
  SharedUnit .so function-pointer fields (on_setup/on_render/on_component/
  on_websocket/on_cli/on_once/on_init) in types.h/types.cpp. compile_shared_unit
  now builds only the .wasm side-module; the .uce preprocessor/parser front-end
  is kept (it emits the C++ the wasm compile consumes).
- unit_call()/component()/once/init now resolve across units through the wasm
  host component resolver (uce_host_component_resolve) instead of native dlsym;
  configured runtime error pages render through the wasm backend.
- Dropped the WASM_BACKEND_ENABLED feature flag and dead COMPILE_SCRIPT /
  COMPILE_WASM_UNITS config; unit ABI freshness tied to UCE_UNIT_ABI_VERSION;
  guard against serving a stale .wasm for a deleted source; retired the obsolete
  W5 native-vs-wasm toggle script. Docs updated.

Implemented via the pi agent (with 3 delegated sub-reviews); independently
re-verified on the build host: run_cli_tests --include-wasm-kill => 87 passed,
0 failed, 0 skipped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/build_linux.sh

@@ -9,11 +9,11 @@ GF="uce_fastcgi"
 mkdir bin > /dev/null 2>&1
 mkdir bin/tmp > /dev/null 2>&1
 mkdir bin/assets > /dev/null 2>&1
+mkdir bin/wasm > /dev/null 2>&1
 mkdir work > /dev/null 2>&1
 
 COMPILER="clang++"
-# -rdynamic is a link-time flag (exports the binary's symbols so dlopen'd .uce
-# units resolve the runtime against it); the -c compiles below do not need it.
+# -rdynamic is a link-time flag; the -c compiles below do not need it.
 FLAGS="-g -w -Wall -$OPT_FLAG -std=c++20 -fpermissive -ffast-math"
 
 # Wasmtime C++ API — needed only by the wasm backend object (src/wasm).
@@ -30,8 +30,8 @@ SRCFLAGS="-D EXEC_NAME=\"$GF\" -D PLATFORM_NAME=\"linux\""
 #   bin/sqlite3.o  vendored SQLite amalgamation (depends only on its own source)
 #   bin/wasm.o     wasm backend + worker + wasmtime.hh (src/wasm)
 #   bin/main.o     linux_fastcgi.cpp + the uce_lib core amalgamation
-# All link into the single -rdynamic binary, so the dlopen unit model is
-# unchanged. Delete bin/*.o to force a clean rebuild.
+# All link into the single -rdynamic binary. Delete bin/*.o to force a clean
+# rebuild.
 
 # Rebuild $1 if it is missing or anything under the remaining find-args is newer.
 needs_rebuild() {
@@ -41,6 +41,14 @@ needs_rebuild() {
 	return 1
 }
 
+# core.wasm: guest runtime loaded by the native wasm backend.
+if needs_rebuild bin/wasm/core.wasm src/wasm/core.cpp src/lib src/wasm/core_hostcalls.syms src/wasm/core_libc_exports.syms scripts/build_core_wasm.sh; then
+	echo "Compiling wasm core..."
+	bash scripts/build_core_wasm.sh || exit 1
+else
+	echo "Reusing bin/wasm/core.wasm"
+fi
+
 # SQLite: vendored C, depends only on its own source (not our headers).
 if needs_rebuild bin/sqlite3.o src/3rdparty/sqlite/sqlite3.c src/3rdparty/sqlite/sqlite3.h; then
 	echo "Compiling SQLite..."