fix(p2-shim): coerce browser file read bounds to numbers

wasi-filesystem read takes filesize (u64) arguments, which the
canonical-ABI binding passes as BigInt. Uint8Array.slice rejects
BigInt and throws "Cannot convert a BigInt value to a number" on
the first read against any file in the browser implementation:

    TypeError: Cannot convert a BigInt value to a number
        at Uint8Array.slice (<anonymous>)
        at Descriptor.read (preview2-shim/lib/browser/filesystem.js:191)

Coerce both arguments to Number on entry. Safe up to
Number.MAX_SAFE_INTEGER bytes (~9 PB), well above any practical
browser use.

Signed-off-by: Zachary Whitley <zachary.whitley@tegmentum.ai>

Author: Zachary Whitley

packages/preview2-shim/lib/browser/filesystem.js

@@ -188,7 +188,9 @@ class Descriptor {
 
   read(length, offset) {
     const source = getSource(this.#entry);
-    return [source.slice(offset, offset + length), offset + length >= source.byteLength];
+    const off = typeof offset === "bigint" ? Number(offset) : offset;
+    const len = typeof length === "bigint" ? Number(length) : length;
+    return [source.slice(off, off + len), off + len >= source.byteLength];
   }
 
   write(buffer, offset) {