Decode JSON from Data without boxing every byte

[rnaud]

JSONDecoder.decoder(from:) parses with JSONParser(bytes: data.bytes). On Android that is expensive in memory:

• Data.bytes is Array(platformValue.map { $0.toUByte() }): the map produces a list of boxed UBytes and Array(...) copies it again.
• JSONParser(bytes:) then rebuilds a Data and a String from that array before handing it to org.json.

That comes to roughly 40 bytes of heap per byte of input before the tokenizer sees a character. We hit this in an app decoding a ~3.6 MB response: a heap dump showed ~5.8 M boxed UBytes plus a ~4 M-slot Object[], and the process ran out of memory on a default 192 MB heap.

Change

• Adds JSONParser.init(data:), which builds the String directly from the data's backing ByteArray (String(data:encoding:)), with the same UTF-8 fallback as init(bytes:).
• JSONDecoder.decoder(from:) uses it. No behaviour change otherwise.
• Adds TestJSON.testJSONDecodingMultiByteAndLargeInput, covering multi-byte UTF-8 and a 20,000-element array.

With the change, a heap dump at the same point shows no boxed UBytes. What remains is the parsed JSON itself.

Not in this PR

JSONSerialization.jsonObject(with:) has the same data.bytes call, but it uses the bytes to detect the encoding (BOM, UTF-16/32), so it needs a slightly larger change: detect from the first few bytes, then convert once. Happy to follow up with that if this approach looks right.

Testing

• swift test --filter SkipFoundationTests.TestJSON/ (macOS): 5 passed.
• swift test --filter XCSkipTests (Robolectric, Java 25): 506 passed, 0 failed, 485 skipped. TestJSON 5/5, including the new test.

🤖 Generated with Claude Code

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[rnaud]

CLA signed in skiptools/clabot-config#101 (with my employer's approval). I'll comment recheck once that's merged.

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