Harden runtime type validation system

[wolfy-j]

• Fix table? validation rejecting valid tables (Ref("table") unresolved from manifest, builtin type fallback added to resolveRuntimeType)
• Add structured validation errors via *Error with Kind=Invalid and details map (field, expected, got, constraint) accessible from Lua through err:kind(), err:details(), err:message()
• Fast-path :is() using zero-alloc validateValue on success, only computing error details on failure (2.6x speedup, 0 allocs on pass)
• Handle all 32 typ.Type implementations in runtime validation: Recursive, Sum, Platform, Intersection, Ref, Annotated nil-inner
• Nil-safe String()/Kind() on all compound types (Record, Map, Array, Tuple, Union, Intersection, Optional, Annotated) to prevent segfaults from corrupted manifests
• Guard all reflection methods (elem, key, val, inner, ret, fields, variants, params) and type comparison against nil internals
• Recursion depth limit (64) on validation to prevent stack overflow from malformed recursive types
• Map validation checks Array part for number-keyed maps
• Record validation checks MapComponent for records with map components
• Type IO decoder hardening: maxSliceLen reduced to 64, depth limit 32, node budget 1024, missing checkSliceLen in readCondition
• CI: add fuzz, race, and benchmark jobs