fix(buffer): Node arg-validation for alloc/allocUnsafe/concat/byteLength (#2013)

[proggeramlug]

What

Extends Node argument-validation parity (#2013) to the global Buffer factory methods, following the fs slice landed in #2035 / #2328. These functions previously coerced bad input silently, so assert.throws-style tests reported "Missing expected exception".

API	Bad input	Now throws
Buffer.alloc / allocUnsafe / allocUnsafeSlow	non-number size	TypeError [ERR_INVALID_ARG_TYPE]
″	NaN / Infinity / negative / > kMaxLength	RangeError [ERR_OUT_OF_RANGE]
Buffer.concat	non-Array list	TypeError [ERR_INVALID_ARG_TYPE]
″	non-Buffer/Uint8Array element	TypeError [ERR_INVALID_ARG_TYPE] (list[i])
″	non-integer / negative totalLength	ERR_INVALID_ARG_TYPE / ERR_OUT_OF_RANGE
Buffer.byteLength	non string/Buffer/ArrayBuffer/TypedArray	TypeError [ERR_INVALID_ARG_TYPE]

Non-integer sizes still truncate toward zero (Buffer.alloc(2.5).length === 2), matching Node.

How

• Reuses the fs::validate primitives (describe_received, throw_type_error_with_code, throw_range_error_with_code, validate_int32) — the issue explicitly names this the shared validation home. describe_received gains string/bigint rendering (Node's determineSpecificType shape) — purely additive; fs never reaches those branches.
• New buffer/validate.rs with js_buffer_validate_size, js_buffer_validate_concat_list, validate_concat_length, validate_byte_length_arg.
• The size/list values reach validation as raw NaN-boxed values: the Buffer factory codegen (env_clones.rs, array_methods.rs) now calls the validators before coercing to i32 / treating as an ArrayHeader (the latter also closes a latent UB where Buffer.concat('x') cast a string pointer to an array header). byteLength / concat(totalLength) validate inside the runtime entry points.
• Buffer.alloc() / allocUnsafe() with no argument now default to undefined so they surface ERR_INVALID_ARG_TYPE like Node, instead of silently allocating empty.

Testing

Verified byte-identical to Node v25 across the full buffer node-suite (120 cases, including 3 new arg-validation tests under allocation/, concat/, byte-length/) — 0 diffs, 0 regressions. cargo fmt --all --check clean.

Scope

#2013 is a cross-cutting umbrella (~85 tests across fs/buffer/net/http/crypto/zlib/process/url). fs landed earlier; this is the buffer slice. Buffer.compare is deferred (it lowers to an instance a.compare(b) dispatch — a separate path); remaining APIs remain follow-ups.