Compiled execution plans and tag introspection

Committed (24a5a6c). Release notes for v0.4.0:

Validation is now compiled once per type into a cached execution plan and executed as pre-bound closures, instead of re-parsing directive strings on every call. All operands are resolved at compile time: length and value bounds, defaults, oneof sets, and regular expressions (whose cache lock is gone from the hot path), while untagged fields and no-op containers are pruned from the plan entirely. Behavior is unchanged, including error messages, mutation ordering, and Validate-method dispatch, and Compile now also warms the plan cache. On a representative nested struct (Apple M1 Pro), Validate drops from 6350 ns and 103 allocations to 1273 ns and 17; a flat 4-field struct drops from 1010 ns and 15 allocations to 161 ns and zero. The release also exports ParseTag, which parses a dv8 tag into its directives without checking validity, enabling tooling such as an OpenAPI generator to project the directives onto other formats while keeping the tag grammar single-sourced.