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.

include/cst_web_md.h

@@ -0,0 +1,48 @@
+#ifndef CST_WEB_MD_H
+#define CST_WEB_MD_H
+
+/* Markdown for the WebUI: the same sashidown renderer the TUI uses, but
+ * HTML output with a strict allowlist.
+ *
+ * sashidown renders straight to HTML and escapes text, quotes and '&', so
+ * no raw markup can reach the browser. Two holes remain, and both matter
+ * because every byte rendered here comes from a model or a tool: a link's
+ * href keeps any scheme ("javascript:") and an image's src may point
+ * anywhere (a tracking pixel, or worse). cst_web_md_* therefore rewrites
+ * tags instead of passing them through: unknown tags are dropped, hrefs
+ * are limited to http/https/mailto/relative, and images to the session's
+ * own attachment endpoint.
+ *
+ * The streaming form renders confirmed HTML as the model talks, which is
+ * the CST property the project is built on: nothing is emitted before it
+ * is safe to emit. A tag split across two deltas is held back until its
+ * '>' arrives. */
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Called with chunks of sanitized HTML; return non-zero to stop. */
+typedef int (*cst_web_md_emit_fn)(const char *html, size_t len, void *ud);
+
+typedef struct cst_web_md cst_web_md_t;
+
+cst_web_md_t *cst_web_md_new(cst_web_md_emit_fn emit, void *ud);
+/* Feed model output. Returns 0, or -1 on OOM. */
+int cst_web_md_feed(cst_web_md_t *s, const char *data, size_t len);
+/* Flush the pending input and close the renderer. */
+int cst_web_md_finish(cst_web_md_t *s);
+void cst_web_md_free(cst_web_md_t *s);
+
+/* One-shot: markdown -> sanitized HTML (malloc'd, never NULL on success). */
+char *cst_web_md_html(const char *md, size_t len);
+/* The sanitizer alone (tests, and any HTML that did not come from here). */
+char *cst_web_md_sanitize(const char *html, size_t len);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* CST_WEB_MD_H */