Add scoped Perl runtimes and isolate I/O state

[fglock]

Summary

• add the empty PerlRuntime identity shell with strict scoped, non-inherited bindings
• bind CLI, provider, JSR-223, require/do, and eval ownership boundaries
• isolate standard handles, selected/last-handle bookkeeping, readline context, and open-handle caching per runtime
• route standard globs and glob localization/deletion through the bound runtime
• explicitly propagate runtime identity into pipe/system/process routers and Netty callbacks
• update the final threads plan and mark Phases 3 and 4 complete

Phases 3 and 4 are intentionally combined in this PR at maintainer direction. All other mutable interpreter state remains process-global, concurrent full Perl execution is not yet advertised, and Config thread flags remain disabled.

Validation

• make — passed
• make test-all — passed; core 83.2%, bundled modules 80.8%
• 24 focused I/O regression runs across JVM and interpreter backends — passed by process status
• Scalar::Util 1.70 and Moo 2.005005 smoke tests — passed on both backends
• IPC::Open3 and standard-glob lifecycle checks — passed on both backends
• git diff --check — passed

The interpreter-only TAP miss in nonblocking_pipe_syswrite.t assertion 8 was reproduced on the exact PR #918 base; it is pre-existing. Its JVM run and the remaining focused assertions pass.

Next phase

After merge, Phase 5 migrates execution, local-save, dynamic-scope, and special-block stacks into PerlRuntime. Perl threads remains unavailable until the later cloning and API activation phases.