HS: intersection observer mock + on intersection (+3 tests)

Applied from worktree-agent-ad6e17cbc4ea0c94b (commit 0a0fe314)
with manual re-apply onto post-cluster-26 HEAD:

- Parser: parse-on-feat collects `having margin X threshold Y`
  clauses between `from X` and the body; packs them into a
  `:having {"margin" M "threshold" T}` dict on the parts list.
- Compiler: scan-on threads a new `having-info` parameter through
  all recursions; when event-name is "intersection", wraps the
  hs-on call with `(do on-call (hs-on-intersection-attach! target
  margin threshold))`.
- Runtime: hs-on-intersection-attach! constructs an
  IntersectionObserver with {rootMargin, threshold} options and a
  callback that dispatches an "intersection" DOM event carrying
  {intersecting, entry} detail.
- Runner: HsIntersectionObserver mock fires the callback
  synchronously on observe() with isIntersecting=true so handlers
  run during activation; ignores margin/threshold (tests assert
  only that the handler fires).

Suite hs-upstream-on: 33/70 -> 36/70 (on intersection: 0/3 -> 3/3).
Smoke 0-195 unchanged at 165/195.

tests/hs-run-filtered.js

@@ -354,7 +354,26 @@ class HsResizeObserver {
   }
 }
 globalThis.ResizeObserver=HsResizeObserver; globalThis.ResizeObserverEntry=class{};
-globalThis.IntersectionObserver=class{observe(){}disconnect(){}};
+// HsIntersectionObserver — cluster-27 intersection mock. Fires the callback
+// synchronously on observe() with isIntersecting=true so `on intersection`
+// handlers run during activation. `margin`/`threshold` options are parsed
+// but ignored (tests only assert the handler fires).
+class HsIntersectionObserver {
+  constructor(cb, opts) { this._cb = cb; this._opts = opts || {}; this._els = new Set(); }
+  observe(el) {
+    if (!el) return;
+    this._els.add(el);
+    const entry = { target: el, isIntersecting: true, intersectionRatio: 1,
+                    boundingClientRect: (el.getBoundingClientRect && el.getBoundingClientRect()) || {},
+                    intersectionRect: {}, rootBounds: null, time: 0 };
+    try { this._cb([entry], this); } catch (e) {}
+  }
+  unobserve(el) { if (el) this._els.delete(el); }
+  disconnect() { this._els.clear(); }
+  takeRecords() { return []; }
+}
+globalThis.IntersectionObserver = HsIntersectionObserver;
+globalThis.IntersectionObserverEntry = class {};
 globalThis.navigator={userAgent:'node'}; globalThis.location={href:'http://localhost/',pathname:'/',search:'',hash:''};
 globalThis.history={pushState(){},replaceState(){},back(){},forward(){}};
 globalThis.getSelection=()=>({toString:()=>(globalThis.__test_selection||'')});