fix: remove design-debt shims — falsy ID bug, selector normalization, concurrency, exception swallowing

pipeline.ts: || undefined → ?? undefined (9 occurrences)
- || converts valid subject ID 0 to undefined because 0 is falsy in JS.
  This broke clause witnesses and topology references for the first subject.

domain-index.ts: remove .toLowerCase() on CSS selectors
- CSS selectors are case-sensitive (IDs, class names, attribute values).
  Lowercasing on lookup but not on storage (selectorIndex) meant case-
  sensitive selectors never matched — returning empty arrays silently.

canonical.ts: add warning when visualBoxes falls back to layout boxes
- visualBoxes ?? boxes silently substituted layout coordinates for visual
  space, producing incorrect results for transform-dependent assertions.
  Now emits console.warn so silent data corruption is visible.

extraction.ts: serialize materializeSemanticSelector calls (3 sites)
- Changed Promise.all over page.evaluate() to sequential for..of. While
  Playwright serializes CDP calls internally, concurrent DOM-modifying
  evaluate() calls create undefined execution order. Sequential resolution
  eliminates theoretical race conditions for semantic selector injection.

engine.ts: include stack trace in evaluator exception diagnostics
- Catch-all converted ALL exceptions (including TypeError from programming
  bugs) to IMH_EVALUATOR_EXCEPTION with just err.message. Now includes
  stack trace and logs to console.warn for visibility. Distinguishes
  TypeError (programming bug) from other evaluation errors.

648 SDK tests + 57 E2E hard tests pass, zero regressions.

packages/imhotep-geometry/src/domain-index.ts

@@ -16,7 +16,7 @@ import { GeometryWorld } from './world.js'
  * If the selector is not indexed, returns an empty array.
  */
 export function getElementsBySelector(world: GeometryWorld, selector: string): number[] {
-  const normalized = selector.trim().toLowerCase()
+  const normalized = selector.trim()
   return world.selectorIndex.get(normalized) ?? []
 }