buffer: fix 32-bit truncation in SlowCopy for buffers larger than 4 GiB

[crawfordxx]

Summary

SlowCopy in src/node_buffer.cc extracted targetStart, sourceStart,
and to_copy via .As<Uint32>()->Value(), silently truncating values that
exceed 2^32 - 1 (4 GiB). This caused Buffer.prototype.copy() to copy
the wrong number of bytes or write to the wrong position when dealing with
buffers larger than 4 GiB.

Root cause was introduced in #54087 ("buffer: use native copy impl"), which
replaced the original ParseArrayIndex-based extraction (using size_t) with
Uint32Value (32-bit).

Changes

• src/node_buffer.cc: Use IntegerValue() (returns int64_t) in SlowCopy
  instead of .As<Uint32>()->Value(), and widen CopyImpl's parameter types
  from uint32_t to size_t.
• FastCopy is left unchanged — V8's Fast API guarantees it is only invoked
  when all arguments fit in uint32_t; for larger values V8 falls back to
  SlowCopy automatically.
• test/parallel/test-buffer-copy-large.js: New test that allocates a buffer
  slightly larger than 4 GiB and verifies Buffer.prototype.copy() copies all
  bytes correctly. The test skips gracefully on systems with insufficient memory.

Test

// Before fix: only 5 bytes were copied (2^32 + 5 mod 2^32 = 5).
// After fix: all FOUR_GB + 5 bytes are copied correctly.
const src = Buffer.alloc(2 ** 32 + 5, 0xbb);
const dst = Buffer.alloc(src.length, 0xaa);
src.copy(dst);
assert.strictEqual(dst[dst.length - 1], 0xbb);

Fixes #55422

[Renegade334]

Don't use checked operations here.

Suggested change

	const size_t target_start =
	static_cast<size_t>(args[2]->IntegerValue(ctx).ToChecked());
	const size_t source_start =
	static_cast<size_t>(args[3]->IntegerValue(ctx).ToChecked());
	const size_t to_copy =
	static_cast<size_t>(args[4]->IntegerValue(ctx).ToChecked());
	CopyImpl(source_obj, target_obj, target_start, source_start, to_copy);
	int64_t target_start, source_start, to_copy;
	if (!args[2]->IntegerValue(env->context()).To(&target_start) ||
	!args[3]->IntegerValue(env->context()).To(&source_start) ||
	!args[4]->IntegerValue(env->context()).To(&to_copy)) {
	return;
	}
	CopyImpl(source_obj,
	target_obj,
	static_cast<size_t>(target_start),
	static_cast<size_t>(source_start),
	static_cast<size_t>(to_copy));

[Renegade334]

Please also change the FastCopy argument and return types from uint32_t to uint64_t.

[Renegade334]

Copying gigabytes of buffer memory is a massive operation. Can this just test copying a small chunk of memory with source/target offsets of >2^32?