chore: delete stale planning docs, update README test count

Remove 4 stale planning markdown files per project rules (No Extra
Markdown Files): PROBLEM_GENERATION_PLAN.md, PROBLEM_GENERATION_TASKS.md,
VISUALIZATION_ASSIGNMENTS.md, VISUALIZATION_PARTITION.md.

Update README test count from 13,745 to 13,687 to match current suite.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: davidagustin

PROBLEM_GENERATION_PLAN.md

@@ -189,7 +189,7 @@ pnpm format           # Format with Biome
 pnpm validate         # typecheck + lint:biome + lint
 
 # Testing
-pnpm test             # Unit (Vitest) — 13,745 tests, ~96% coverage
+pnpm test             # Unit (Vitest) — 13,687 tests
 pnpm test:coverage    # Unit with coverage report
 pnpm test:e2e        # E2E (Playwright)
 pnpm test:e2e:ui     # Playwright UI