feat: covariant self-returns, use-const/enum-name parse edges, exception $previous, unknown-array foreach keys

[chadmandoo]

Five bounded completeness fixes found compiling real strict-typed PHP:

• INTERFACE COVARIANCE: an implementation may return a narrower type than the interface declares (the PSR-7 withX(): static shape). type_accepts can never see it — the class under validation is not yet registered mid-construction — so conformance accepts a self-typed return from its own context: the validated interface (or one it extends), or any interface the class declares (or one those extend).
• PARSE: use const PHP_INT_MAX; — the lexer eagerly tokenizes such constants; the use-declaration prefix accepts the dedicated tokens by canonical spelling (imports are inert — expression uses lower to literals). class Enum / new Enum / Vendor\Enum — enum is a soft keyword (vendor precedent: marc-mabe/php-enum); parse_name and the class-decl name accept it.
• EXCEPTIONS: builtin exception constructors accept PHP's third $previous parameter (positional or named). Not stored — the compact throwable payload has no previous slot and getPrevious() is already synthesized as null — wrap-and-rethrow code compiles with byte-identical message/code round-trips.
• FOREACH KEYS: iterating an unknown-element array (array-hinted param/property) types keys as Mixed, not Int — the value may be associative at runtime.

Byte-parity vs PHP 8.5 on all five regression tests.