gc diagnostics: per-tier heap pressure surfaced in repl tabbar

Step 1 of the GC effort. Each tier loader now exposes heapStats():
  - asm-wasm — lumbda_heap_used / lumbda_heap_total wat exports
  - c-wasm   — emscripten linear memory size (no free path right now,
               so used = total; documented in the loader)
  - python   — pyodide module linear memory size; CPython GC cycles
               this naturally

Worker handles a "heap" message kind that round-trips the active tab's
loaded tiers; repl tabbar shows a compact "py 12M · c 32M · asm 4M"
strip next to the buttons. Polls every 2s.

Doesn't solve the leak — just makes pressure visible so the user knows
when to use "reboot tier". Real GC (Cheney over the WAT bump allocator,
Boehm-em or custom mark-sweep for c-wasm) coming next.

wasm/python/lumbda-py.js

@@ -56,6 +56,13 @@ def _lumbda_eval(src):
             pyodide.globals.set("_src_in", src);
             return await pyodide.runPythonAsync("_lumbda_eval(_src_in)");
         },
+        heapStats() {
+            // Pyodide's runtime memory is the Emscripten linear memory.
+            // CPython's GC reclaims behind the scenes, so this number
+            // rises and falls naturally as objects die.
+            const total = pyodide._module.HEAPU8.byteLength;
+            return { used: total, total };
+        },
     };
 }