refactor: add #[\Override], make psalm pass without ext-msgpack

[roxblnfk]

🔍 What was changed

• Every method that overrides or implements a parent/interface method in src/ carries #[\Override], except MsgpackCodec, which is left untouched until the msgpack deprecation is decided. On PHP 8.2 the attribute is inert, no polyfill added.
• Psalm now loads the existing resources/stubs/msgpack.phpstub, so it passes without ext-msgpack installed (before, it reported UndefinedFunction for msgpack_pack/msgpack_unpack locally).
• The baseline shrinks to the MsgpackCodec closure-type entries only: Frame::initFrame() now handles a false from unpack(), and the port/stream checks in SocketRelay and StreamRelay carry inline @psalm-suppress DocblockTypeContradiction — they are runtime guards for callers without static analysis, so removing them would be a behaviour change.
• No public API changes: no classes made final, no signatures changed (ClassMustBeFinal stays disabled in psalm.xml).

Stacked on #40.

Checklist

• How was this tested:
  • vendor/bin/psalm (6.19.2) passes locally; also passes with --php-version=8.3
  • Testo suite run locally against the Go test server: same results as on feat: migrate tests to Testo, require PHP 8.2 #40

[coderabbitai]

✨ Finishing Touches

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