v0.7.8: WebUI (--web): HTTP/1.1 + SSE front end, engine API v1, tests

A second implementation of cst_agent_hooks_t next to the TUI: the turn runs on
a worker thread, the hooks publish events into a ring buffer that SSE clients
read, so the engine itself needs no changes.

- src/net.c, include/cst_net.h: POSIX/Winsock sockets, non-blocking connect
- src/http_srv.c, include/cst_http.h: HTTP/1.1, keep-alive, chunked streaming,
  body caps (413), Expect: 100-continue, connection cap, exact routing
- src/web_md.c, include/cst_web_md.h: streaming markdown -> sanitized HTML
  (tag/attribute allowlist, javascript: URLs and off-site images dropped)
- src/webui.c, include/cst_webui.h: event ring + SSE broadcast, all routes,
  token (Bearer/Cookie/query) and loopback Host checks, 3 s auto-deny when no
  client is attached
- src/webui_stub.c, CST_WEBUI=0 / CST_BUILD_WEBUI=OFF: build without the WebUI
- web/: no-build front end (transcript, approvals, todo, sessions, image
  upload, status bar)
- docs/webui.md: engine API v1 - events, requests, security model
- tests/test_webui.c, scripts/e2e_webui.sh: unit tests plus an end-to-end run
  driven by a recorded SSE stream (no API key needed)
- src/main.c: --web --web-port --web-host --web-token --web-dev --web-open
  and CST_WEB_PORT/HOST/TOKEN/ROOT

An empty POST body is now accepted without Content-Length (RFC 9112 6.3) so
curl-driven clients can reach cancel/undo/compact/quit; CMakeList's core tests
learned about sashidown and test_webui, and CST_WEBUI=0 defines CST_NO_WEBUI.

Makefile

@@ -43,6 +43,10 @@ ifeq ($(strip $(CURL_LIBS)),)
 CURL_LIBS := -lcurl
 endif
 
+# WebUI (HTTP + SSE front end): src/net.c src/http_srv.c src/web_md.c
+# src/webui.c. CST_WEBUI=0 swaps in the stub (--web then prints a hint).
+CST_WEBUI ?= 1
+
 PKG_CFLAGS := $(CURL_CFLAGS) $(CURSES_CFLAGS)
 PKG_LIBS := $(CURL_LIBS) $(CURSES_LIBS)
 
@@ -50,6 +54,17 @@ CPPFLAGS += -Iinclude -I.gen -I$(SD_DIR)/include -I$(CJSON_DIR) -D_XOPEN_SOURCE_
 LDFLAGS += -L$(SD_DIR)
 LDLIBS += -lsashidown -lm $(PKG_LIBS) $(PTHREAD_LIBS)
 
+WEB_DIR := web
+WEB_ASSETS := $(wildcard $(WEB_DIR)/*.html) $(wildcard $(WEB_DIR)/*.js) \
+	$(wildcard $(WEB_DIR)/*.css) $(wildcard $(WEB_DIR)/*.svg)
+ifeq ($(CST_WEBUI),0)
+WEB_SRC := src/net.c src/http_srv.c src/web_md.c src/webui_stub.c
+# The stub body is guarded by this, so the switch has to define it too.
+CPPFLAGS += -DCST_NO_WEBUI=1
+else
+WEB_SRC := src/net.c src/http_srv.c src/web_md.c src/webui.c
+endif
+
 CORE_SRC = \
 	$(CJSON_DIR)/cJSON.c \
 	src/util.c \
@@ -79,12 +94,13 @@ CORE_SRC = \
 	src/creds.c \
 	src/mm_tool.c \
 	src/models.c src/model_registry.c \
-	src/metrics.c
+	src/metrics.c \
+	$(WEB_SRC)
 
 SRC = $(CORE_SRC) src/textsel.c src/tui.c src/main.c
 OBJ = $(SRC:.c=.o)
 HDRS = $(wildcard include/*.h) $(wildcard src/tools/*.h) $(CJSON_DIR)/cJSON.h \
-	.gen/models_builtin.inc
+	.gen/models_builtin.inc .gen/web_assets.inc
 
 TESTS = \
 	tests/test_ctl_frontier \
@@ -120,8 +136,12 @@ TESTS = \
 	tests/test_win_utf8 \
 	tests/test_os_dir \
 	tests/test_tmpdir
+# test_webui drives the real server; there is nothing to test with the stub.
+ifneq ($(CST_WEBUI),0)
+TESTS += tests/test_webui
+endif
 
-TEST_LIBS = -lm $(CURL_LIBS) $(PTHREAD_LIBS)
+TEST_LIBS = -L$(SD_DIR) -lsashidown -lm $(CURL_LIBS) $(PTHREAD_LIBS)
 
 .PHONY: all clean check demo sashidown-lib FORCE cmake-gcc cmake-clang asan install uninstall
 
@@ -153,6 +173,11 @@ cst-agent: $(OBJ) $(SD_DIR)/libsashidown.a
 	@mkdir -p .gen
 	od -An -v -tx1 $< | sed 's/\([0-9a-f][0-9a-f]\)/0x\1,/g' > $@.tmp && mv $@.tmp $@
 
+# The WebUI shell is embedded the same way, one array per file.
+.gen/web_assets.inc: $(WEB_ASSETS) scripts/embed_web.sh
+	@mkdir -p .gen
+	@sh scripts/embed_web.sh $@ $(WEB_DIR)
+
 %.o: %.c $(HDRS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<
 
@@ -186,10 +211,14 @@ tests/test_markdown: tests/test_markdown.c $(SD_DIR)/libsashidown.a
 tests/test_%: tests/test_%.c $(CORE_SRC) $(HDRS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $< $(CORE_SRC) $(TEST_LIBS)
 
+$(TESTS): sashidown-lib
+
 check: $(TESTS)
 	@set -e; for t in $(TESTS); do echo "== $$t"; ./$$t; done
 
 # AddressSanitizer + UBSan build of the agent and all tests.
+# Under a ptrace-based sandbox (some Termux setups) LeakSanitizer aborts with
+# "Tracer caught signal 11"; add ASAN_OPTIONS=detect_leaks=0 there.
 asan:
 	$(MAKE) clean
 	$(MAKE) sashidown-lib
@@ -199,7 +228,7 @@ demo: cst-agent
 	./cst-agent --demo-ctl --workspace .
 
 clean:
-	rm -f $(OBJ) cst-agent $(TESTS) .gen/models_builtin.inc
+	rm -f $(OBJ) cst-agent $(TESTS) .gen/models_builtin.inc .gen/web_assets.inc
 
 install: cst-agent
 	install -d "$(DESTDIR)$(PREFIX)/bin"