feat: add wasm build targets, profile hooks, os_demo example, and restructure manual source

- Add `wasm`, `wasm-clean`, and `install-emscripten` phony targets to Makefile for WebAssembly cross-compilation
- Insert `WREN_PROFILE_ENTER`/`WREN_PROFILE_EXIT` macros in `wren_vm.h` and `wren_vm.c` to support optional runtime profiling via `WREN_PROFILE_ENABLED`
- Create `example/os_demo.wren` demonstrating Platform, Process, and conditional exit usage
- Update `example/regex_demo.wren` to import `Match` and exercise Match object properties, groups, and `matchAll`
- Remove static HTML manual pages (`base64.html`, `dns.html`, `json.html`) and replace with structured `manual_src/` directory containing Jinja2 templates, YAML metadata, and content pages
- Expand `README.md` with build targets table, manual building instructions, source layout, and guide for adding new module documentation

Makefile

@@ -1,6 +1,6 @@
 # retoor <retoor@molodetz.nl>
 
-.PHONY: build tests clean debug sync-sidebar buildmanual
+.PHONY: build tests clean debug sync-sidebar buildmanual wasm wasm-clean install-emscripten
 
 build:
 	cd projects/make && $(MAKE) -f wren_cli.make -j $$(nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || echo 4)
@@ -26,5 +26,27 @@ install:
 clean:
 	cd projects/make && $(MAKE) -f wren_cli.make clean
 
+wasm-clean:
+	cd projects/make.wasm && $(MAKE) -f wren_wasm.make clean
+
+install-emscripten:
+	@if [ ! -d "$(HOME)/emsdk" ]; then \
+		echo "Cloning Emscripten SDK..."; \
+		git clone https://github.com/emscripten-core/emsdk.git $(HOME)/emsdk; \
+		cd $(HOME)/emsdk && ./emsdk install latest && ./emsdk activate latest; \
+		echo ""; \
+		echo "Emscripten installed. Run: source ~/emsdk/emsdk_env.sh"; \
+	else \
+		echo "Emscripten SDK already exists at ~/emsdk"; \
+		echo "Run: source ~/emsdk/emsdk_env.sh"; \
+	fi
+
+wasm:
+	@if [ -z "$$EMSDK" ]; then \
+		echo "Error: EMSDK is not set. Run 'source ~/emsdk/emsdk_env.sh' first"; \
+		exit 1; \
+	fi
+	cd projects/make.wasm && $(MAKE) -f wren_wasm.make -j $$(nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || echo 4)
+
 buildmanual:
 	python3 util/build_manual.py