Optimize JSON serializer

[colemancda]

Summary

Rewrites the serializer core to write into a [UInt8] buffer instead of building a String and re-walking it. Public API (toString / toData / SerializationOptions) and output are unchanged and byte-identical. All techniques are Embedded-safe.

Independent of the parser work in #4 (different file), so the two can merge in any order.

Changes

• Byte-buffer core — toData() returns the accumulated [UInt8] directly; toString() wraps it in String(decoding:). Previously toData() built a String then re-encoded it to UTF-8 — two full traversals.
• Bulk string escaping — escape-free runs (the common case, including object keys) are appended in one append(contentsOf:) over the source UTF-8, replacing the per-unicodeScalar append.
• Integer formatting without allocation — signed/unsigned integers are written digit-by-digit into a stack buffer (withUnsafeTemporaryAllocation), replacing .description String allocations. Int64.min handled via unsigned negation.
• Static hex table — the \u00XX control-escape table is a static [UInt8] instead of a [Character] array rebuilt on every string.

Performance (1.10 MB output / 5000 objects, release, Apple Silicon)

Serializer	Throughput
master toData()	~47 MB/s
this branch toData()	~164 MB/s
this branch toString()	~157 MB/s
reference	~200 MB/s

~3.5× faster; the remaining 1.22× vs the reference is its buffer capacity pre-estimate (a recursive size walk we chose not to add — diminishing returns for the extra pass).

Verification

• 51 tests pass (added integer 0 / min / max, empty and escaped string serialization cases)
• Coverage 99.89%, serializer file 100%
• wasm-embedded release build clean

[github-code-quality]

Code Coverage Overview

Languages: Swift

Swift / code-coverage/llvm-cov

The overall coverage in the feature/serializer-p... branch is 100%. The coverage in the master branch is 99%.

Show a code coverage summary of the most impacted files.

File	master e8f9204	feature/serializer-p... a146328	+/-
Sources/JSONMac...ONCodable.swift	100%	0%	-100%
Sources/JSONMacros/Error.swift	100%	0%	-100%
Sources/JSON/Serializer.swift	100%	100%	0%
Sources/JSONMac...os/Macros.swift	0%	0%	0%
Sources/JSON/Parser.swift	99%	100%	+1%

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