Grouped emission walks the entries slice in three passes instead of copying them into per-kind slices, error paths render their key path only when an error names it, inline tables are measured once because a measuring encoder no longer nests another measurement, clean string runs are written in one write, and date-times render through the single-buffer path. The output buffer comes from a sync.Pool and returns to it within a 1 MiB retention cap, the output handed to the caller as a copy; repeated marshals keep the live heap flat, verified over 500 runs. The encoder resolves Marshaler and the text interfaces through the same per-type flag cache and hint the decoder uses, and docs/ARCHITECTURE.md now lists the caches and the pool as the library's shared state.
Representative document: 177 to 141 allocations, 12 230 to 6170 bytes per operation; long document: 97 175 to 63 660 allocations, 6.97 to 2.79 MB per operation, about 7.8 to about 3.4 ms.
Per-statement work no longer allocates where it does not need to: parseKeyPath returns the first key component directly, absolute paths and key buffers sit on the parser, the definition maps are created on first write, arrays presize, and a string without escapes is copied out in one piece instead of built byte by byte. Repeated keys share one string across array-of-tables elements through a parser-local intern table whose lookup works on the bytes, so a repeated key costs no allocation.
UTF-8 validity is no longer a whole-input pass before the parser runs: the scan validates where it meets a multi-byte sequence, and a SyntaxError now names the line the invalid byte sits on. TestParseRejectsInvalidUTF8 covers nine positions; docs/API.md describes the new reporting.
Long document: 67 664 to 31 765 allocations per parse, about 51 to about 106 MB/s; representative document: 157 to 104 allocations.
The suite gains BenchmarkStrictDecodeLong and BenchmarkMarshalLong over the same 2000-entry document ParseLong uses, so long-input work is visible on every path, not the parse alone. docs/BENCHMARKING.md lists them.
petrbalvin