feat(runtime): swallow() helper replaces empty catches in inlined runtime

After hf#641 inlined the runtime IIFE into every bundle, lint tools
inspecting bundled output (including Abhay's c2v eval) started flagging
empty `catch {}` blocks across the runtime. The source had explanatory
comments inside, but esbuild's minifier strips them — the IIFE ships
~10 visible patterns of `}catch{}` and consumers' linters fire on each.

Each empty catch is intentional best-effort error swallowing —
postMessage to a parent frame that may not exist, `media.play()` /
`pause()` that throw under autoplay restrictions, timeline `seek()` on
a disposed timeline, anime.js / lottie feature detection on hosts that
don't load those libraries, etc. The right behaviour stays "tried,
didn't work, move on", but doing it visibly improves three things:

  - lint clean: helper call is a real statement; no `no-empty` warnings
    survive minification
  - debuggable: flip `window.__hfDebug = true` in DevTools to see every
    swallow site with `console.debug` (silent in prod by default)
  - observable: studio / embeddings can install
    `window.__hf.onSwallowed = handler` to collect runtime swallow
    events without polluting the page console

Implementation: `packages/core/src/runtime/diagnostics.ts` exports
`swallow(label, err?)`. 41 catch sites across 12 runtime files
converted via mechanical pass (auto-generated `runtime.<module>.siteN`
labels — labels can be tightened site-by-site as a follow-up; the
shape of the change is what matters here).

Verification:
- core 674/674 (incl. 6 new diagnostics tests covering silent default,
  __hfDebug logging, legacy __HYPERFRAMES_DEBUG flag, handler hook,
  handler-throws-doesn't-recurse, both-active)
- typecheck clean
- format / lint clean
- runtime IIFE rebuilds successfully (`bun run build:hyperframes-runtime`)

Refs Abhay's c2v eval — bundler artefacts now lint-clean with the
runtime body inlined.

packages/core/src/runtime/diagnostics.test.ts

@@ -0,0 +1,77 @@
+// @vitest-environment happy-dom
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { swallow } from "./diagnostics";
+
+interface HFTestWindow {
+  __hfDebug?: boolean;
+  __HYPERFRAMES_DEBUG?: boolean;
+  __hf?: {
+    onSwallowed?: (e: { label: string; error: unknown }) => void;
+  };
+}
+
+describe("swallow", () => {
+  const w = window as unknown as HFTestWindow;
+  const originalDebug = console.debug;
+
+  beforeEach(() => {
+    delete w.__hfDebug;
+    delete w.__HYPERFRAMES_DEBUG;
+    delete w.__hf;
+    console.debug = vi.fn();
+  });
+
+  afterEach(() => {
+    console.debug = originalDebug;
+    delete w.__hfDebug;
+    delete w.__HYPERFRAMES_DEBUG;
+    delete w.__hf;
+  });
+
+  it("is silent by default — no console output, no handler call", () => {
+    swallow("test.silent", new Error("boom"));
+    expect(console.debug).not.toHaveBeenCalled();
+  });
+
+  it("logs to console.debug when window.__hfDebug is true", () => {
+    w.__hfDebug = true;
+    const err = new Error("boom");
+    swallow("test.debug", err);
+    expect(console.debug).toHaveBeenCalledWith("[hyperframes] test.debug swallowed:", err);
+  });
+
+  it("also honors window.__HYPERFRAMES_DEBUG (legacy flag)", () => {
+    w.__HYPERFRAMES_DEBUG = true;
+    swallow("test.legacy", "string-error");
+    expect(console.debug).toHaveBeenCalledWith(
+      "[hyperframes] test.legacy swallowed:",
+      "string-error",
+    );
+  });
+
+  it("dispatches to window.__hf.onSwallowed when installed", () => {
+    const handler = vi.fn();
+    w.__hf = { onSwallowed: handler };
+    const err = new Error("from handler");
+    swallow("test.handler", err);
+    expect(handler).toHaveBeenCalledWith({ label: "test.handler", error: err });
+  });
+
+  it("does not propagate errors from the user-installed handler", () => {
+    w.__hf = {
+      onSwallowed: () => {
+        throw new Error("handler exploded");
+      },
+    };
+    expect(() => swallow("test.handler-throws", new Error("real"))).not.toThrow();
+  });
+
+  it("can run with both handler AND debug flag set", () => {
+    w.__hfDebug = true;
+    const handler = vi.fn();
+    w.__hf = { onSwallowed: handler };
+    swallow("test.both", "err");
+    expect(handler).toHaveBeenCalled();
+    expect(console.debug).toHaveBeenCalled();
+  });
+});