pyvm: split op handlers into ops_core/ops_box/ops_ctrl; add ops_flow + intrinsic; delegate vm.py without behavior change

net-plugin: modularize constants (consts.rs) and sockets (sockets.rs); remove legacy commented socket code; fix unused imports
mir: move instruction unit tests to tests/mir_instruction_unit.rs (file lean-up); no semantic changes
runner/pyvm: ensure using pre-strip; misc docs updates

Build: cargo build ok; legacy cfg warnings remain as before

docs/guides/testing-matrix.md

@@ -0,0 +1,26 @@
+# Testing Matrix — Mapping Specs to Tests
+
+Purpose
+- Map invariants/constraints to the concrete tests (smokes/goldens/unit) that verify them.
+
+Categories
+- PHI hygiene (LLVM)
+  - ir_phi_empty_check.sh — no empty PHIs
+  - ir_phi_hygiene_if_phi_ret.sh — PHIs at block head with if/ret pattern
+- MIR hints (VM)
+  - hints_trace_smoke.sh — basic scope enter/leave
+  - hints_join_result_* — join diagnostics for 2/3 vars
+  - hints_scope_trycatch_smoke.sh — try/catch scopes
+- Match normalization (VM/goldens)
+  - match_literal_basic / literal_three_arms output smokes
+  - match_guard_literal_or / type_basic_min goldens
+- Exceptions (VM)
+  - expr_postfix_catch_cleanup_output_smoke.sh — postfix direct parser
+  - loop_postfix_catch_cleanup_output_smoke.sh — combined with loops
+- LoopForm break/continue (VM)
+  - loopform_continue_break_output_smoke.sh — basic continue/break
+  - loop_nested_if_ctrl_output_smoke.sh — nested if inside loop
+  - loop_nested_block_break_output_smoke.sh — nested bare block with break
+
+Maintenance
+- When adding an invariant or lifting a constraint, update this matrix and link the tests.