security: replace denylist sanitize_share_html with allowlist el_html_sanitize

A real attacker probed /api/share earlier today with <script>alert(1),
<iframe src=evil>, <img onerror>, <a href="javascript:...">, and a
<form action="/steal"> payload. Nothing executed because the chat
bubble at /share/<id> renders the served HTML inside marked.js's
already-escaped output, but the prior denylist sanitizer was fragile:

  - It comment-wrapped dangerous tags ("<!--script>...-->") which a
    literal "-->" inside an attacker-supplied attribute value can close
    early, re-exposing the original payload.
  - It renamed on*= attributes to data-x-on*= which left attack
    indicators visible in the served HTML.
  - It was a denylist; every new attack vector required a code change.
  - It didn't validate <a href> URL schemes properly.

The replacement is a runtime-level state-machine allowlist parser
(foundation/el af480f6: el_html_sanitize). The product just specifies
the JSON allowlist of allowed tags + attributes; the runtime drops
everything else, validates href/src URL schemes (http/https/mailto/
fragment/relative only), and drops whole subtrees of script/style/
iframe/object/embed/form regardless of the allowlist.

Phase 4 of bl-dc55ae07: deletes sanitize_share_html (main.el) and
gal_sanitize_html (gallery.el); replaces 3 call sites with
el_html_sanitize(html, allowlist). Defines default_share_allowlist
in main.el and the identical gallery_share_allowlist in gallery.el
(separate bindings to avoid a forward-reference at build-concat
order — gallery is concatenated before main).

Phase 5: migrations/20260502185500_backfill_resanitize_share_cards.sql
nulls answer_html for any share_cards row older than 1 hour. Applied
via the Supabase Management API; 0 rows in scope (the column was
added today and existing rows pre-date its first write).

Also fixes an orthogonal duplicate-symbol bug: unix_timestamp() was
defined in both dist/web_stubs.c and the runtime (the latter is a
recent runtime addition picked up by the runtime sync). Removed the
stub.

Backlog: bl-dc55ae07

runtime/el_runtime.h

@@ -199,6 +199,19 @@ el_val_t  http_get_to_file(el_val_t url, el_val_t headers_map, el_val_t output_p
 el_val_t  url_encode(el_val_t s);   /* RFC 3986 unreserved set */
 el_val_t  url_decode(el_val_t s);   /* '+' → space, %XX → byte  */
 
+/* ── HTML allowlist sanitizer ────────────────────────────────────────────────
+ * el_html_sanitize(input_html, allowlist_json) — strict allowlist HTML
+ * cleaner. State-machine parser; tag/attribute names compared case-
+ * insensitively against the allowlist; `<a href>` / `<… src>` URL schemes
+ * validated (http, https, mailto, fragment-only, or relative); whole-
+ * subtree drop for script / style / iframe / object / embed / form; HTML-
+ * escapes free text outside dropped subtrees.
+ *
+ * The allowlist is JSON of the form
+ *     {"p":[],"a":["href","title"],"strong":[],...}
+ * where each value is the array of attribute names allowed for that tag. */
+el_val_t  el_html_sanitize(el_val_t input_html, el_val_t allowlist_json);
+
 /* ── Filesystem ──────────────────────────────────────────────────────────── */
 
 el_val_t  fs_read(el_val_t path);
@@ -246,6 +259,63 @@ el_val_t  time_from_parts(el_val_t secs, el_val_t ns, el_val_t tz);
 el_val_t  time_add(el_val_t ts, el_val_t n, el_val_t unit);
 el_val_t  time_diff(el_val_t ts1, el_val_t ts2, el_val_t unit);
 
+/* ── Instant + Duration: first-class temporal types ──────────────────────────
+ * Both types share the el_val_t (int64) slot. Instants are nanoseconds
+ * since the Unix epoch; Durations are signed nanoseconds. Type discipline
+ * is enforced at codegen-time: BinOps on names registered as Instant or
+ * Duration route through the typed wrappers below; mismatches like
+ * Instant+Instant become #error at the C compiler.
+ *
+ * Postfix literals — `30.seconds`, `1.hour`, `500.millis`, `30.nanos` — are
+ * recognised by the parser as DurationLit AST nodes and lowered to literal
+ * int64 nanoseconds at codegen time. The runtime never sees the units. */
+
+el_val_t  el_now_instant(void);
+el_val_t  now(void);
+el_val_t  unix_seconds(el_val_t n);
+el_val_t  unix_millis(el_val_t n);
+el_val_t  instant_from_iso8601(el_val_t s);
+
+el_val_t  el_duration_from_nanos(el_val_t ns);
+el_val_t  duration_seconds(el_val_t n);
+el_val_t  duration_millis(el_val_t n);
+el_val_t  duration_nanos(el_val_t n);
+
+el_val_t  el_instant_add_dur(el_val_t inst, el_val_t dur);
+el_val_t  el_instant_sub_dur(el_val_t inst, el_val_t dur);
+el_val_t  el_instant_diff(el_val_t a, el_val_t b);
+el_val_t  el_duration_add(el_val_t a, el_val_t b);
+el_val_t  el_duration_sub(el_val_t a, el_val_t b);
+el_val_t  el_duration_scale(el_val_t dur, el_val_t scalar);
+el_val_t  el_duration_div(el_val_t dur, el_val_t scalar);
+
+el_val_t  el_instant_lt(el_val_t a, el_val_t b);
+el_val_t  el_instant_le(el_val_t a, el_val_t b);
+el_val_t  el_instant_gt(el_val_t a, el_val_t b);
+el_val_t  el_instant_ge(el_val_t a, el_val_t b);
+el_val_t  el_instant_eq(el_val_t a, el_val_t b);
+el_val_t  el_instant_ne(el_val_t a, el_val_t b);
+el_val_t  el_duration_lt(el_val_t a, el_val_t b);
+el_val_t  el_duration_le(el_val_t a, el_val_t b);
+el_val_t  el_duration_gt(el_val_t a, el_val_t b);
+el_val_t  el_duration_ge(el_val_t a, el_val_t b);
+el_val_t  el_duration_eq(el_val_t a, el_val_t b);
+el_val_t  el_duration_ne(el_val_t a, el_val_t b);
+
+el_val_t  instant_to_unix_seconds(el_val_t i);
+el_val_t  instant_to_unix_millis(el_val_t i);
+el_val_t  instant_to_iso8601(el_val_t i);
+el_val_t  duration_to_seconds(el_val_t d);
+el_val_t  duration_to_millis(el_val_t d);
+el_val_t  duration_to_nanos(el_val_t d);
+
+el_val_t  el_sleep_duration(el_val_t dur);
+el_val_t  unix_timestamp(void);
+
+el_val_t  ttl_cache_set(el_val_t key, el_val_t value);
+el_val_t  ttl_cache_get(el_val_t key, el_val_t max_age);
+el_val_t  ttl_cache_age(el_val_t key);
+
 /* ── UUID ────────────────────────────────────────────────────────────────── */
 
 el_val_t  uuid_new(void);
@@ -288,7 +358,7 @@ el_val_t  str_char_at(el_val_t s, el_val_t i);
 el_val_t  str_char_code(el_val_t s, el_val_t i);
 el_val_t  str_pad_left(el_val_t s, el_val_t width, el_val_t pad);
 el_val_t  str_pad_right(el_val_t s, el_val_t width, el_val_t pad);
-el_val_t  str_format(el_val_t template, el_val_t data);
+el_val_t  str_format(el_val_t fmt, el_val_t data);
 el_val_t  str_lower(el_val_t s);
 el_val_t  str_upper(el_val_t s);