feat: add multi-engine regex flavour support

engines/go/README.md

@@ -0,0 +1,26 @@
+# 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.