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b1f7a5cc49
Release-mode benchmarks, UTIME word, consolidated promotion
Three changes:
1. Add UTIME host function ( -- ud ) for microsecond timing in Forth.
Enables self-timed benchmarks matching gforth's utime approach.
2. Switch comparison benchmarks to release mode: builds wafer binary
with --release, measures via UTIME (excludes startup overhead).
Previously measured debug-mode Rust overhead, not WASM execution.
3. Add stack-to-local promotion to consolidated codegen path. Words
that pass is_promotable now use the StackSim emit path even in
CONSOLIDATE'd modules, preventing performance regression.
Release-mode results (WAFER beats gforth on 4/5 benchmarks):
Factorial: 0.54x (2x faster)
GCD: 0.50x (2x faster)
NestedLoops: 0.10x (10x faster)
Collatz: 0.31x (3x faster)
Fibonacci: 1.47x (call overhead)