Record promoted optimizer costs and validation

Author: CryZe

TODO.md

@@ -1010,7 +1010,7 @@ Implement in this order:
    functions, runtime helpers, imports, types, static data, and scratch regions
    can disappear. Integrate this with existing demand-driven emission instead
    of maintaining a second dependency model.
-5. [ ] Verify equivalent observable behavior with focused runtime tests and the
+5. [x] Verify equivalent observable behavior with focused runtime tests and the
    maintained corpus, confirm Debug emission is unaffected, and record measured
    byte savings and compiler resource costs. Enable the proven passes only for
    Release. Add further passes only when size measurements justify them.

docs/WASM_SIZE_OPTIMIZATION.md

@@ -56,6 +56,25 @@ fits into the existing instruction scan. A second cleanup sweep saves another
 enabled; it exposes useful simplifications without introducing another analysis.
 Pass savings interact and must not be added together as independent totals.
 
+## Compiler cost
+
+Optimized-host seven-sample medians for the promoted pipeline:
+
+| Real script | Passes disabled | Promoted Release backend | Added backend time |
+| --- | ---: | ---: | ---: |
+| Minish Cap | 2.978 ms | 6.448 ms | 3.470 ms |
+| Lunistice | 16.505 ms | 19.161 ms | 2.656 ms |
+| Neon White | 0.505 ms | 1.005 ms | 0.500 ms |
+| A Hat in Time | 4.556 ms | 7.469 ms | 2.913 ms |
+
+These measurements include lowering and emission but exclude parsing/type
+checking. Previous full-experiment totals were 7.591 / 20.856 ms on the primary
+scripts. The selected pipeline is faster in these measurements while retaining
+almost all size savings; cross-run differences are not isolated per-pass costs.
+All nine fixtures and individual samples are in `target/size-check/timing.json`.
+The optimized-host corpus check also validates identical sizes to the native
+development build. Debug does not pay these optimizer costs.
+
 ## Validation and reproduction
 
 The promoted pipeline has focused instruction/control/function-sharing tests