Optimize int loop jumps and document loop var immutability

notes/nested_loop_vm_state.md

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+## Nested loop performance investigation state (2026-02-15)
+
+### Key findings
+- Bytecode for `NestedRangeBenchmarkTest` is fully int-local ops; no dynamic lookups or scopes in hot path.
+- Loop vars now live directly in local int slots (`n1..n6`), removing per-iteration `MOVE_INT`.
+- Per-instruction try/catch in VM was replaced with an outer try/catch loop; on JVM this improved the benchmark.
+- Native slowdown is likely dominated by suspend/virtual dispatch overhead in VM, not allocations in int ops.
+
+### Current bytecode shape (naiveCountHappyNumbers)
+- Ops: `CONST_INT`, `CMP_GTE_INT`, `INC_INT`, `ADD_INT`, `CMP_EQ_INT`, `JMP*`, `RET`.
+- All are `*Local` variants hitting `BytecodeFrame` primitive arrays.
+
+### Changes made
+- Loop vars are enforced read-only inside loop bodies at bytecode compile time (reassign, op=, ?=, ++/-- throw).
+- Range loops reuse the loop var slot as the counter; no per-iteration move.
+- VM loop now uses outer try/catch (no per-op try/catch).
+- VM stats instrumentation was added temporarily, then removed for MP safety.
+
+### Files changed
+- `lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt`
+  - loop var immutability checks
+  - loop var slot reuse for range loops
+  - skip break/result init when not needed
+- `lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt`
+  - outer try/catch VM loop (removed per-op try/catch)
+  - stats instrumentation removed
+- `lynglib/src/commonTest/kotlin/NestedRangeBenchmarkTest.kt`
+  - temporary VM stats debug removed
+  - slot dump remains for visibility
+- `notes/bytecode_vm_notes.md`
+  - note: opcode switch dispatch tested slower than virtual dispatch
+
+### Benchmark snapshots (JVM)
+- Before VM loop change: ~96–110 ms.
+- With VM stats enabled: ~234–240 ms (stats overhead).
+- After VM loop change, stats disabled: ~85 ms.
+- 2026-02-15 baseline (fused int-compare jumps): 74 ms.
+  - Command: `./gradlew :lynglib:jvmTest -Pbenchmarks=true --tests '*NestedRangeBenchmarkTest*'`
+  - Notes: loop range checks use `JMP_IF_GTE_INT` (no CMP+bool temp).
+
+### Hypothesis for Native slowdown
+- Suspend/virtual dispatch per opcode dominates on K/N, even with no allocations in int ops.
+- Next idea: a non-suspend fast path for hot opcodes, or a dual-path VM loop.