Add Zig language support module (rewrite-zig)

[zieka]

Summary

Adds Zig language support to OpenRewrite following the rewrite-go module architecture:

• Java side: AST model (Zig.java with 9 node types), ZigVisitor, ZigParser, RPC codecs, formatting visitors, services
• Zig subprocess: RPC server using std.zig.Ast for parsing, cursor-based whitespace extraction, LST serialization, and source printer
• Rich AST mapping: Functions, const/var declarations, identifiers, literals, binary/unary ops, if/else, while loops, switch expressions, blocks, return, defer, test decls, comptime, error unions, slices, field access, method calls, assignments, array access, try/catch/orelse
• 36 integration tests with structural assertions verifying real AST types (not just round-trip)
• 13 Zig-side unit tests wired into ./gradlew check via zigTest task

Known gaps

• for loops map to J.Unknown (Zig's multi-input/payload syntax doesn't fit J.ForEachLoop.Control)
• Container declarations (struct/enum/union bodies) map to J.Unknown
• No type attribution (Zig stdlib doesn't expose a type-checker API)
• allowNonWhitespaceInWhitespace validation escape hatch (same as rewrite-go)
• Import management returns no-op visitor (gracefully skips Zig files)

Test plan

• ./gradlew :rewrite-zig:zigTest — 13 Zig-side native tests
• ./gradlew :rewrite-zig:integTest — 36 Java integration tests (round-trip + structural)
• ./gradlew :rewrite-zig:check — full verification including license headers