Cardano Updates

- Exclude crates/uplc-fuzz from workspace (broken dep after rebase)
- Add bench pre-check to skip scripts that fail decode/eval
- Add utility examples: eval_flat, eval_flat_bc, check_max_var,
  bc_dump, bench_loop (for profiling)

Three optimizations to the VM dispatch loop:

1. Replace unbudgeted_steps[9] total counter with a countdown that
   decrements to zero. Saves one array increment per step and turns
   the >= comparison into a free zero-check after decrement.

2. Restructure the run loop to drain return chains without going back
   through the outer compute loop. Eliminates Phase construction and
   matching on the hot Compute→Return transition.

3. Box the CasesFrame (rare, 0.3% of opcodes) to shrink Frame from
   32 to 16 bytes, halving memory bandwidth for every frame push/pop.

256 nested lambdas with a reference to the outermost variable,
exercising the VarBig opcode. Without VarBig, the u8 index silently
wraps and the VM looks up the wrong variable.

- Expand builtin pre-wrap range from 0..92 to 0..101 for 9 new builtins
  (Value ops, MultiScalarMul)
- Add case_on_constant handler: Bool (with branch count validation),
  Unit, Integer, List (head/tail field pushing), Pair (fst/snd)
- Add VarBig opcode (0x16) with u32 index for De Bruijn indices > 255,
  fixing silent truncation that caused wrong variable lookups in large
  scripts (e.g. stablecoin contracts)
- Compiler emits Var (u8) for indices <= 255, VarBig (u32) otherwise;
  ApplyVar/ForceVar superinstructions restricted to u8 range

The rvcas/value-builtins branch added new builtins that used the old
runtime.args[n] field access. Updated to use runtime.arg(n) accessor
(fixed-size array with unsafe unchecked access). Also fixed blst_p1/p2
pointer casts in MultiScalarMul builtins.