Go regexp WebAssembly engine

This directory is the fixed bridge around Go 1.26.5's standard-library regexp package. It is compiled with the official go1.26.5 toolchain for GOOS=js GOARCH=wasm; public/engines/go/wasm_exec.mjs must come from that same toolchain.

The bridge accepts JSON data only. The adapter measures the exact encoded request size, and the native cap includes JSON's worst-case sixfold escaping for every accepted string field. It implements native RE2-family Go syntax, native UTF-8 byte offsets, SubexpNames, Find(All)StringSubmatchIndex, and Regexp.ExpandString replacement templates. Application flags i, m, s and U become Go inline flags; g selects native global iteration.

Replacement uses a bounded streaming implementation of Regexp.ExpandString grammar. Load-time differential fixtures compare it with the native API, and capture references stop expanding as soon as the UTF-8 output cap is reached. The bounded bytes cross the JSON bridge as base64, so control-heavy output has a fixed 4:3 transport expansion instead of JSON's content-dependent escaping.

Go deliberately excludes look-around and backreferences. Its global iterator also ignores an empty match that abuts the preceding match. Those behaviours are tested rather than emulated.

The bridge independently caps pattern, subject, match, capture-row and output sizes. Replacement output is clipped only at a complete UTF-8 boundary.