refactor: Major interpreter modularization and P2PBox enhancements

Major Interpreter Refactoring:
- Split core.rs (373 lines removed) into focused modules
- Split expressions/calls.rs (460 lines removed) into cleaner structure
- Added new modules: calls.rs, errors.rs, eval.rs, methods_dispatch.rs, state.rs
- Improved separation of concerns across interpreter components

P2PBox Enhancements:
- Added on_once() for one-time event handlers
- Added off() for handler deregistration
- Implemented handler flags with AtomicBool for thread-safe management
- Added loopback testing cache (last_from, last_intent_name)
- Improved Arc-based state sharing for transport and handlers

Plugin Loader Unification (In Progress):
- Created plugin_loader_unified.rs skeleton
- Created plugin_ffi_common.rs for shared FFI utilities
- Migration plan documented (2400 lines → 1100 lines target)

MIR & VM Improvements:
- Enhanced modularized MIR builder structure
- Added BoxCall dispatch improvements
- Better separation in builder modules

Documentation Updates:
- Added Phase 9.79a unified box dispatch plan
- Created plugin loader migration plan
- Updated CURRENT_TASK.md with latest progress

All tests passing (180 tests) - ready for next phase of refactoring

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/mod.rs

@@ -11,6 +11,8 @@ pub mod instruction_introspection; // Introspection helpers for tests (core inst
 pub mod basic_block;
 pub mod function;
 pub mod builder;
+#[cfg(feature = "mir_modular_builder")]
+pub mod builder_modularized; // Modular split of MIR builder (optional)
 pub mod loop_builder; // SSA loop construction with phi nodes
 pub mod loop_api;     // Minimal LoopBuilder facade (adapter-ready)
 pub mod verification;
@@ -25,7 +27,10 @@ pub use instruction::{MirInstruction, BinaryOp, CompareOp, UnaryOp, ConstValue,
 pub use instruction_v2::{MirInstructionV2, AtomicOrdering}; // New 25-instruction set
 pub use basic_block::{BasicBlock, BasicBlockId, BasicBlockIdGenerator};
 pub use function::{MirFunction, MirModule, FunctionSignature};
+#[cfg(not(feature = "mir_modular_builder"))]
 pub use builder::MirBuilder;
+#[cfg(feature = "mir_modular_builder")]
+pub use builder_modularized::MirBuilder;
 pub use verification::{MirVerifier, VerificationError};
 pub use ownership_verifier_simple::{OwnershipVerifier, OwnershipError, OwnershipStats}; // Simple ownership forest verification
 pub use printer::MirPrinter;