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>

src/wasm/worker.cpp

@@ -1046,6 +1046,9 @@ private:
 	// __uce_<base>[_<suffix>] for a handler spec like "component:CARD" / "render".
 	static String handler_export_symbol(const String& handler)
 	{
+		String export_prefix = "export:";
+		if(handler.rfind(export_prefix, 0) == 0)
+			return(handler.substr(export_prefix.size()));
 		auto colon = handler.find(":");
 		String symbol = "__uce_" + (colon == String::npos ? handler : handler.substr(0, colon));
 		if(colon != String::npos)
@@ -1086,6 +1089,9 @@ private:
 			return(1);
 		}
 
+		if(!file_exists_host(worker.unit_wasm_path(resolved)) || compiler_unit_needs_recompile(context, resolved, 0))
+			get_shared_unit(context, resolved, true);
+
 		size_t unit_index = 0;
 		String error = load_unit(resolved, unit_index);
 		if(error != "")
@@ -1791,7 +1797,7 @@ private:
 				return(std::monostate());
 			}));
 		if(mod == "env" && name == "uce_host_component_resolve")
-			return(add([self](Caller, Span<const Val> args, Span<Val> results) -> Result<std::monostate, Trap> {
+			return(add([self](Caller caller, Span<const Val> args, Span<Val> results) -> Result<std::monostate, Trap> {
 				String target, handler, current, resolved;
 				self->hostcall_read(args[0].i32(), args[1].i32(), target);
 				self->hostcall_read(args[2].i32(), args[3].i32(), handler);
@@ -1806,6 +1812,7 @@ private:
 					self->hostcall_write(args[6].i32(), resolved);
 				}
 				results[0] = Val(slot);
+				caller.context().set_epoch_deadline(self->worker.cfg.epoch_deadline_ticks);
 				return(std::monostate());
 			}));