test(filter): cover pipeline coordinator + core filters

internal/filter (the 31-layer compression pipeline) was 24%: added tests for
the real entry points — PipelineCoordinator.Process across ModeNone/Minimal/
Aggressive on code/prose/JSON/empty/large inputs (stats, budget enforcement,
determinism, idempotence, guardrail fallback, nil-receiver guard), plus the
composed core filters (GoalDriven, Contrastive, Ngram, EvaluatorHeads, Gist,
HierarchicalSummary) and CoreFilters.Apply. Skipped the experimental
auto-generated layer files. filter 24->28%; tok overall 37->40%. Gate 36->38.

Author: Patel230

.github/workflows/ci.yml

@@ -105,7 +105,7 @@ jobs:
       - name: Coverage threshold
         run: |
           COVERAGE=$(go tool cover -func=coverage.out | tail -1 | grep -oE '[0-9]+\.[0-9]+' || echo "0")
-          THRESHOLD=36
+          THRESHOLD=38
           if [ "$(echo "$COVERAGE < $THRESHOLD" | bc -l)" -eq 1 ]; then
             echo "::error::Coverage ${COVERAGE}% is below threshold ${THRESHOLD}%"
             exit 1