Add switchable optimization config and benchmark framework

WaferConfig: unified config controlling all optimizations individually.
ForthVM::new_with_config(config) to create VMs with custom optimization settings.
All 8 switchable optimizations: peephole, constant_fold, strength_reduce, dce,
tail_call, inline (IR passes) + stack_to_local_promotion (codegen).

Benchmark framework (crates/core/tests/benchmark_report.rs):
- 7 Forth benchmarks: Fibonacci, Factorial, SumRecurse, NestedLoops, GCD, MemFill, Collatz
- Correctness verification across all configs (runs in CI)
- Full report with 128 optimization combinations (cargo test --ignored)
- Measures execution time, compilation time, WASM module bytes
- CONSOLIDATE impact comparison

Key findings from benchmark report:
- Inlining: -77% exec time on Fibonacci, -92% on Collatz
- Stack-to-local promotion: -5.5% WASM module size
- CONSOLIDATE: -72% exec time on Fibonacci (call_indirect -> direct call)
- All optimizations combined: best overall performance

crates/core/src/config.rs

@@ -0,0 +1,61 @@
+//! Unified configuration for all WAFER optimizations.
+
+use crate::optimizer::OptConfig;
+
+/// Codegen-level optimization flags.
+#[derive(Debug, Clone)]
+pub struct CodegenOpts {
+    /// Enable stack-to-local promotion for straight-line words.
+    pub stack_to_local_promotion: bool,
+}
+
+/// Master configuration for all WAFER optimizations.
+#[derive(Debug, Clone)]
+pub struct WaferConfig {
+    /// IR-level optimization passes.
+    pub opt: OptConfig,
+    /// Codegen-level optimizations.
+    pub codegen: CodegenOpts,
+}
+
+impl WaferConfig {
+    /// All optimizations enabled.
+    pub fn all() -> Self {
+        Self {
+            opt: OptConfig {
+                peephole: true,
+                constant_fold: true,
+                tail_call: true,
+                strength_reduce: true,
+                dce: true,
+                inline: true,
+            },
+            codegen: CodegenOpts {
+                stack_to_local_promotion: true,
+            },
+        }
+    }
+
+    /// All optimizations disabled.
+    pub fn none() -> Self {
+        Self {
+            opt: OptConfig {
+                peephole: false,
+                constant_fold: false,
+                tail_call: false,
+                strength_reduce: false,
+                dce: false,
+                inline: false,
+            },
+            codegen: CodegenOpts {
+                stack_to_local_promotion: false,
+            },
+        }
+    }
+}
+
+impl Default for WaferConfig {
+    fn default() -> Self {
+        Self::all()
+    }
+}