fix(runtime): unknown method on a Buffer receiver must throw, not return undefined

[proggeramlug]

Problem

dispatch_buffer_method's catch-all returns undefined for any method that neither the Buffer API arms nor the delegated %TypedArray%.prototype tower implement. Node throws:

const buf = readFileSync(path);   // Buffer since #1420 (no encoding → Buffer, Node parity)
buf.charCodeAt(0);                // Node: TypeError: buf.charCodeAt is not a function
                                  // Perry: undefined — and execution continues

Plain objects, strings, and number receivers already throw through js_throw_type_error_not_a_function; Buffer was the one receiver that swallowed the call.

Why it matters

The silence has real teeth precisely because PerryTS#1420 correctly aligned readFileSync with Node: any caller still treating the no-encoding result as a string now gets undefined from every string-method call and keeps running on garbage instead of failing at the call site. Real-world case (how we found it): a code editor's NUL-byte binary-detection scan — content.charCodeAt(i) === 0 over the first 8000 chars — misclassified every file it opened as binary. No crash, no error message, nothing to grep for; the app just quietly stopped displaying files. A thrown TypeError would have named the exact line on the first file opened.

This also matches the direction of PerryTS#6453 ("String method on a nullish receiver must throw, not coerce") — loud beats lenient when the alternative is silent corruption.

Fix

In the catch-all, after Buffer-API arms and typed-array delegation both miss, route through js_throw_type_error_not_a_function — the same thrower the string/number primitive catch-alls use, so message shape ((Buffer).charCodeAt is not a function) and catchability match those paths. Internal __perry_* duck-type probes (using-disposal) keep the non-throwing undefined.

Caller audit: both dynamic-dispatch entry points (handle_methods.rs:138, collection_methods.rs:313) return Some(dispatch_buffer_method(...)) unconditionally — nothing relied on the undefined to fall through to another dispatcher. The named-method callers (set/export/slice) never reach the catch-all.

Validation

• New gap test test_gap_buffer_unknown_method_throws.ts: byte-identical to the Node oracle locally (assertions avoid raw message text since V8 renders the callee source expression; the test checks instanceof TypeError + method-name/is not a function substrings, and that execution continues after a caught throw).
• Buffer sweep unchanged: test_buffer_prototype_methods, test_buffer_numeric_read_intrinsic, test_buffer_small_alloc, test_edge_buffer_from_encoding all still match Node. test_compat_buffers_typed diffs on toSorted/toReversed (u8 element widening) — pre-existing, byte-identical before/after this change.

Per CONTRIBUTING, no version/CHANGELOG edits — happy to adjust anything.

[proggeramlug]

Misfiled — this repo is a fork; the canonical repo is PerryTS/perry. Re-submitted there as PerryTS#6464 (rebased onto current main).