node:buffer: coerce Blob and File constructor inputs like Node

[andrewtdiz]

Summary

The existing node:buffer Blob/File implementation covers basic string parts, binary parts, metadata, and object URLs, but the constructors do not apply Node/WHATWG coercion rules for non-binary parts or File metadata. Direct new Blob(...) / new File(...) behavior diverges even though the surface exists.

Node Behavior

Local Node v25.9.0 probe:

(async () => {
  const { Blob, File } = require("node:buffer");
  const parts = [123, true, false, null, undefined, { toString(){ return "obj"; } }, ["a", "b"]];
  const b = new Blob(parts);
  console.log("blob text", JSON.stringify(await b.text()));
  console.log("blob size", b.size);
  const f = new File(parts, 123, { type: "TEXT/PLAIN;Charset=UTF-8", lastModified: "42.9" });
  console.log("file name", JSON.stringify(f.name));
  console.log("file type", JSON.stringify(f.type));
  console.log("file lm", f.lastModified);
  console.log("file text", JSON.stringify(await f.text()));
})();

Observed Node behavior:

• new Blob(parts).text() resolves to "123truefalsenullundefinedobja,b" and size is 31.
• Non-Blob/non-BufferSource parts are stringified, including arrays via normal array ToString (["a", "b"] becomes "a,b"), not recursively flattened as raw parts.
• new File(parts, 123, ...) coerces the name to "123".
• type: "TEXT/PLAIN;Charset=UTF-8" is normalized to "text/plain;charset=utf-8".
• lastModified: "42.9" is coerced to numeric 42.9.

Perry Behavior

Current source evidence:

• crates/perry-codegen/src/lower_call/builtin.rs lowers new Blob(parts, opts) to js_blob_new(parts, type) and new File(parts, name, opts) to js_file_new(parts, name, type, lastModified).
• crates/perry-stdlib/src/fetch_blob.rs::append_blob_part_bytes() only handles strings, Buffer/Uint8Array, Blob handles, and arrays. The comment says anything else is silently dropped, and the array path recurses into elements, so ["a", "b"] becomes "ab" rather than Node's "a,b" stringification.
• js_file_new() only reads name and type when the inputs are heap string tags; non-string names become "" instead of ToString(name).
• js_blob_new() / js_file_new() store type as provided rather than applying Blob type normalization.
• js_file_new() treats lastModified.is_nan() as “use Date.now()”. NaN-boxed non-number values such as a string option take that path instead of ToNumber(lastModified).

Suggested PR Cut

Batch this with related issues in:
node:buffer: Blob and File constructor coercion parity cut

Good batch candidates:

• no other open node:buffer Blob/File coercion leaf is currently known; keep this as a focused PR unless another constructor-coercion issue appears

Do not batch with:

• closed basic Blob/File/object URL support Implement node:buffer Blob, File, and object URL parity #1211
• closed object URL validation node:buffer: validate Blob object URL creation like Node #2982
• node:fs: implement fs.openAsBlob file-backed Blob API #2535 node:fs.openAsBlob() file-backed Blob API
• unrelated Buffer byte/numeric APIs or global File exposure

Acceptance

• parity/regression test proves non-binary Blob parts use Node-compatible ToString / USVString behavior, including arrays becoming comma-joined strings
• parity/regression test proves File name coercion matches Node for non-string names
• Blob/File type normalization matches Node for valid mixed-case MIME strings and invalid characters
• lastModified uses Node-compatible numeric coercion and only defaults to current time when the option is absent
• existing Blob/File binary parts, metadata, text/arrayBuffer behavior, and object URL fixtures continue passing
• related known-failure/docs/manifest entries updated if touched

[andrewtdiz]

Related module-shape work is grouped in #3387 (node:buffer: module namespace and backing property parity cut). Keep #2959 scoped to Blob/File constructor coercion unless implementation overlap makes a shared PR more practical.