docs: point the self-debugging research at the shipped ZDebug package

[lisachenko]

docs/self-debugging.md still spoke of the debugger as a package that was expected to live somewhere else, and its roadmap listed work that has since landed in ZDebug. This actualizes the three places where the study has been overtaken by its own implementation.

• Intro. "The debugger itself is expected to live in a separate package" → it does: ZDebug, which implements this study.
• Statement-hook cost mitigations. Three were listed as "design sketches for the debugger package, not shipped features". The middle one — memoizing the include/exclude decision per op_array address instead of running a backtrace filter per hit — is shipped as ZDebug's OpArrayGate. The compiler-option gating and the NOP strip-back stay sketches.
• Roadmap. The debugger-package list (breakpoint table, suspend loop, DBGp transport, per-file instrumentation gating) is replaced by what ZDebug ships and the three items still open there: the async break command over Core::setInterruptHandler(), the EXT_STMT-vs-ticks benchmark, and the generator/fiber stepping experiment. The z-engine follow-up candidates are unchanged except for adding a zend_ast_process hook for per-file EXT_STMT emission — and the leaner statement-hook fast path stays listed, since OpCodeHook::handle() still runs debug_backtrace() per hit.

The README's self-debugging section also now links ZDebug next to the research it came from.

Docs only — no source, test or CI changes.

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