v0.5.1

v0.5.1 — string fast paths (json_quote / sanitize_expect_value) and operator notes on sweep hook serialisation and child-log rotation

Perf batch #72 (S). Clean values (no '"', '', 0x01-0x1f) skip the per-byte
loop in json_quote and sanitize_expect_value; output is byte-identical to
v0.5.0 (differential test against the frozen tests/fixtures/sitter.baseline).
64 KiB --text: 4.69 s -> 0.075 s. Four remaining perf findings are closed as
measured won't-fix on #72; two of them add operator notes to docs/reference*.md.

Evidence: PR #78 completion record (L1-7), CI on bbb0766 green on ubuntu and
macOS — #78
Issue: #72