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-solver/src/engine.ts

@@ -272,7 +272,13 @@ export function evaluate(
         diagnostics.push(...result.diagnostics);
       }
     } catch (err) {
-      const message = err instanceof Error ? err.message : String(err);
+      const message = err instanceof Error ? err.message : String(err)
+      const stack = err instanceof Error ? err.stack : undefined
+      console.warn(`[imhotep-solver] clause evaluator exception for kind "${clause.clauseKind}" (id ${clause.clauseId}): ${message}`)
+      if (stack) console.warn(stack)
+      const detail = err instanceof Error && err.name === 'TypeError'
+        ? `UNCAUGHT PROGRAMMING BUG: ${message}\n${stack ?? ''}`
+        : `Evaluator error: ${message}`
       const result: ClauseResult = {
         clauseId: clause.clauseId,
         status: 'error',
@@ -282,7 +288,7 @@ export function evaluate(
             code: 'IMH_EVALUATOR_EXCEPTION',
             severity: 'error',
             category: 'internal-error',
-            message,
+            message: detail,
             clauseId: clause.clauseId,
           },
         ],