docs: multiplicity v2 incremental re-implementation plan

[fglock]

Summary

Adds a detailed incremental re-implementation plan for multiplicity after PR #480 was reverted (PR #487).

Root cause of the revert

RuntimeRegex.matchRegexWithTimeout() spawns executor threads for alarm-based regex timeout. After the multiplicity migration, these threads had no PerlRuntime bound via ThreadLocal, causing IllegalStateException when accessing per-runtime regex state. This broke CPAN::Module::parse_version -> MM->parse_version('jar:PERL5LIB/Scalar/Util.pm') -> Moo tests.

What this plan changes

Instead of one 96-file PR, the work is broken into 16 independently testable phases:

• Phase 1: Thread-safety fixes (AtomicInteger counters) — minimal risk
• Phases 2-10: Incremental state migration (I/O, CallerStack, DynamicScope, regex, InheritanceResolver, GlobalVariable, RuntimeCode, WarningBits, ByteCodeSourceMapper)
• Phase 11: COMPILE_LOCK for compilation thread safety
• Phase 12: Thread binding for all background threads (the specific fix for the revert)
• Phase 13: Per-runtime CWD + PID
• Phase 14: Performance optimization (Tier 1-3 from multiplicity-opt branch)
• Phase 15: JvmClosureTemplate + eval STRING fixes
• Phase 16: Demo + documentation

Each phase has its own validation gate: make + make test-all + Scalar::Util version check.

Reference branches

• feature/multiplicity — original implementation (diff target for each phase)
• feature/multiplicity-opt — performance optimizations
• feature/multiplicity-v2 — post-revert fixes

Test plan

• make passes
• Documentation-only change, no code behavior affected

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