fix(runtime): pass model through to the LLM API instead of dropping it

llm_call_system / llm_call accepted a model argument and discarded it:
they called llm_chain_call(system, user) with no model, and the legacy
ANTHROPIC_API_KEY fallback passed NULL to llm_provider_request, so every
non-agentic chat was pinned to LLM_DEFAULT_MODEL (claude-sonnet-4-5)
regardless of the caller's selection.

Thread model_pref through llm_chain_call: provider-chain entries still
honor their own NEURON_LLM_N_MODEL override and fall back to the
requested model otherwise; the legacy Anthropic path now uses the
requested model. NULL/empty preserves prior default behavior.

Effect: the soul's model selection (state soul_model / SOUL_LLM_MODEL,
e.g. claude-opus-4-8) now reaches api.anthropic.com. Previously the
chat response echoed the selected model in its label while the request
billed Sonnet 4.5.

Not built locally (no elc/cc toolchain on this checkout); needs stage CI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lang/el-compiler/runtime/el_runtime.c

@@ -3135,23 +3135,49 @@ static void jb_puts(JsonBuf* b, const char* s) {
 
 static void jb_emit_escaped(JsonBuf* b, const char* s) {
     jb_putc(b, '"');
-    for (; *s; s++) {
-        unsigned char c = (unsigned char)*s;
+    const unsigned char* p = (const unsigned char*)s;
+    while (*p) {
+        unsigned char c = *p;
         switch (c) {
-            case '"':  jb_puts(b, "\\\""); break;
-            case '\\': jb_puts(b, "\\\\"); break;
-            case '\b': jb_puts(b, "\\b");  break;
-            case '\f': jb_puts(b, "\\f");  break;
-            case '\n': jb_puts(b, "\\n");  break;
-            case '\r': jb_puts(b, "\\r");  break;
-            case '\t': jb_puts(b, "\\t");  break;
+            case '"':  jb_puts(b, "\\\""); p++; break;
+            case '\\': jb_puts(b, "\\\\"); p++; break;
+            case '\b': jb_puts(b, "\\b");  p++; break;
+            case '\f': jb_puts(b, "\\f");  p++; break;
+            case '\n': jb_puts(b, "\\n");  p++; break;
+            case '\r': jb_puts(b, "\\r");  p++; break;
+            case '\t': jb_puts(b, "\\t");  p++; break;
             default:
                 if (c < 0x20) {
                     char tmp[8];
                     snprintf(tmp, sizeof(tmp), "\\u%04x", c);
                     jb_puts(b, tmp);
-                } else {
+                    p++;
+                } else if (c < 0x80) {
                     jb_putc(b, (char)c);
+                    p++;
+                } else {
+                    /* Multi-byte UTF-8: validate sequence, pass through if valid,
+                     * escape as \u00xx if the start byte is invalid/orphaned. */
+                    int seq_len = 0;
+                    if      ((c & 0xE0) == 0xC0) seq_len = 2;
+                    else if ((c & 0xF0) == 0xE0) seq_len = 3;
+                    else if ((c & 0xF8) == 0xF0) seq_len = 4;
+                    if (seq_len >= 2) {
+                        int valid = 1;
+                        for (int i = 1; i < seq_len; i++) {
+                            if ((p[i] & 0xC0) != 0x80) { valid = 0; break; }
+                        }
+                        if (valid) {
+                            for (int i = 0; i < seq_len; i++) jb_putc(b, (char)p[i]);
+                            p += seq_len;
+                            break;
+                        }
+                    }
+                    /* Invalid start byte or truncated sequence — escape it */
+                    char tmp[8];
+                    snprintf(tmp, sizeof(tmp), "\\u%04x", c);
+                    jb_puts(b, tmp);
+                    p++;
                 }
                 break;
         }
@@ -8447,7 +8473,7 @@ static el_val_t llm_provider_request(const char* url, const char* key,
     }
 }
 
-static el_val_t llm_chain_call(const char* system_str, const char* user_str) {
+static el_val_t llm_chain_call(const char* model_pref, const char* system_str, const char* user_str) {
     char url_key[64], key_key[64], fmt_key[64], model_key[64];
     for (int i = 0; i < LLM_MAX_PROVIDERS; i++) {
         snprintf(url_key,   sizeof(url_key),   "NEURON_LLM_%d_URL",    i);
@@ -8460,6 +8486,7 @@ static el_val_t llm_chain_call(const char* system_str, const char* user_str) {
         const char* fmt_s = getenv(fmt_key);
         int fmt = (fmt_s && strcmp(fmt_s, "anthropic") == 0) ? 1 : 0;
         const char* model = getenv(model_key);
+        if (!model || !*model) model = model_pref; /* fall back to the caller-requested model */
         fprintf(stderr, "[llm] trying provider %d (%s)\n", i, url);
         el_val_t result = llm_provider_request(url, key, fmt, model, system_str, user_str);
         const char* t = EL_CSTR(result);
@@ -8470,7 +8497,7 @@ static el_val_t llm_chain_call(const char* system_str, const char* user_str) {
     const char* api_key = getenv("ANTHROPIC_API_KEY");
     if (!api_key || !*api_key) return http_error_json("no LLM providers configured");
     fprintf(stderr, "[llm] using legacy ANTHROPIC_API_KEY fallback\n");
-    return llm_provider_request(LLM_API_URL, api_key, 1, NULL, system_str, user_str);
+    return llm_provider_request(LLM_API_URL, api_key, 1, model_pref, system_str, user_str);
 }
 
 /* Legacy llm_request — kept for backward compat with agentic loop internals */
@@ -8534,14 +8561,16 @@ static el_val_t llm_extract_text(el_val_t resp_val) {
 }
 
 el_val_t llm_call(el_val_t model, el_val_t prompt) {
+    const char* m = EL_CSTR(model);
     const char* u = EL_CSTR(prompt); if (!u) u = "";
-    return llm_chain_call(NULL, u);
+    return llm_chain_call(m, NULL, u);
 }
 
 el_val_t llm_call_system(el_val_t model, el_val_t system_prompt, el_val_t user_prompt) {
+    const char* m = EL_CSTR(model);
     const char* s = EL_CSTR(system_prompt); if (!s) s = "";
     const char* u = EL_CSTR(user_prompt);   if (!u) u = "";
-    return llm_chain_call(s, u);
+    return llm_chain_call(m, s, u);
 }
 
 /* ── Tool registry for llm_call_agentic ─────────────────────────────────── */