Execution-only benchmark: AST vs Bytecode VM

Command:

cargo bench --bench execute_expression

Method:

Parse each expression once to AST.
Compile the AST once to bytecode Program.
Benchmark execution only:
- ast.exec(&mut ctx)
- bytecode::execute_program(&program, &mut ctx)
Create a fresh context per iteration for both paths.

Results (Criterion, median)

Scenario	AST exec	Bytecode exec	Delta (Bytecode vs AST)
short_expression	597.55 ns	594.13 ns	-0.57%
long_expression	2.5106 µs	2.5698 µs	+2.36%
function_call	695.27 ns	655.10 ns	-5.78%
list_map_mix	864.45 ns	679.70 ns	-21.37%
ternary_chain	493.75 ns	494.15 ns	+0.08%

At pure execution level, bytecode is not uniformly slower.
Bytecode is close to parity for short-expression and ternary-chain cases in this run.
Bytecode is slower in the long arithmetic-heavy case in this run.
Bytecode is faster in function-call and list/map construction cases in this run.
The previous end-to-end slowdown mostly comes from the extra compile stage per call in execute() (parse + compile + run).