fix: implement CodeRabbit findings (Phase 2)

- Fix prisma-db.md: clarify schema.prisma as source of truth for CHECK constraints
- Fix spelling: "Parametrized" → "Parameterized" in testing-e2e.md and testing.md
- Update median.ts to use MIN_VOTES_FOR_STATISTICS constant instead of hardcoded 3
- Fix globalThis.location state leak in vote_grade.test.ts by saving/restoring
- Simplify getBaseUrl() by removing unreachable 'http://localhost' fallback
- Move task_grade.test.ts from src/test/utils/ to src/lib/utils/ (adjacent pattern)
- Add src/lib/**/*.test.ts to vite.config.ts test.include
- Add 3 new testing patterns to rules/testing.md (multi-location tests, globalThis mocking, guard clause reachability)

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

Author: KATO-Hiro

.claude/rules/prisma-db.md

@@ -98,7 +98,7 @@ user   User @relation(fields: [userId], references: [id])
 
 ## DB-Level Value Constraints
 
-Add `CHECK` constraints (via manual migration SQL) for count and invalid enum values. Document with inline `schema.prisma` comments (e.g., `/// CHECK: count >= 0`) — `prisma-erd-generator` overwrites `ERD.md` on each migration.
+Add `CHECK` constraints via manual migration SQL. Document with inline `schema.prisma` comments (e.g., `/// CHECK: count >= 0`). Note: `prisma-erd-generator` may overwrite `ERD.md` on each migration—always keep the constraint definition in `schema.prisma` as the source of truth.
 
 ## Service Layer Error Handling
 
@@ -125,9 +125,10 @@ Prisma does not support `@@check`. To add one:
 
 1. `pnpm exec prisma migrate dev --create-only --name <description>` — generate migration without applying
 2. Edit the generated `migration.sql` to add the CHECK constraint manually
-3. `pnpm exec prisma migrate dev` — apply
+3. Add an inline comment in `schema.prisma` (e.g., `/// CHECK: constraint description`)
+4. `pnpm exec prisma migrate dev` — apply
 
-Document the constraint in `prisma/ERD.md` (the only place it's visible):
+For visibility, also document complex constraints in `prisma/ERD.md`:
 
 ```mermaid
 %% XOR constraint: workbookplacement_xor_grade_category — exactly one of taskGrade or solutionCategory must be non-null

.claude/rules/testing-e2e.md

@@ -34,7 +34,7 @@ test.describe('logged-in user', () => {
 });
 ```
 
-## Parametrized Tests
+## Parameterized Tests
 
 Playwright has no native `test.each`. Use `for...of` loops (official pattern):
 

.claude/rules/testing.md

@@ -75,7 +75,7 @@ const mockGetUser = (statusCode, user?) => {
 };
 ```
 
-### Parametrized Tests
+### Parameterized Tests
 
 Test enum boundaries + typical value, then separate test for distinct behavior:
 
@@ -115,3 +115,50 @@ Target: 80% lines, 80% branches. Run `pnpm coverage`.
 ## Test Files Ship with Code
 
 Never defer tests. For non-trivial logic without explicit test requirement, add them anyway.
+
+## Multiple Test Location Patterns
+
+During migration, support both centralized (`src/test/`) and co-located (`src/features/`, `src/lib/`) tests.
+Configure `vite.config.ts` with explicit ordering:
+
+```typescript
+include: [
+  'src/lib/**/*.test.ts',        // shared utilities (adjacent)
+  'src/test/**/*.test.ts',       // legacy centralized
+  'src/features/**/*.test.ts',   // feature co-location
+],
+```
+
+## Mocking globalThis Properties
+
+Save and restore `globalThis` state to prevent test leaks:
+
+```typescript
+const original = globalThis.location;
+
+beforeEach(() => {
+  Object.defineProperty(globalThis, 'location', {
+    value: { origin: 'http://test' },
+    writable: true,
+  });
+});
+
+afterEach(() => {
+  if (original !== undefined) {
+    Object.defineProperty(globalThis, 'location', { value: original, writable: true });
+  }
+});
+```
+
+## Guard Clause Reachability
+
+Ensure guard clauses don't make later code unreachable. Example anti-pattern:
+
+```typescript
+// Bad: 'http://localhost' is unreachable
+if (location?.origin) return location.origin;
+if (!browser) return '';
+return 'http://localhost'; // Never reached in browser
+```
+
+Simplify to remove dead code after the final guard.

src/features/votes/internal_clients/vote_grade.test.ts

@@ -10,6 +10,8 @@ import * as statusCodes from '$lib/constants/http-response-status-codes';
 const BASE_URL = 'http://localhost';
 const TASK_ID = 'abc_a';
 
+const originalLocation = globalThis.location;
+
 beforeEach(() => {
   nock.cleanAll();
   // Set globalThis.location for test environment
@@ -21,6 +23,13 @@ beforeEach(() => {
 
 afterEach(() => {
   nock.cleanAll();
+  // Restore original location to prevent test state leak
+  if (originalLocation !== undefined) {
+    Object.defineProperty(globalThis, 'location', {
+      value: originalLocation,
+      writable: true,
+    });
+  }
 });
 
 describe('fetchMyVote', () => {

src/features/votes/internal_clients/vote_grade.ts

@@ -1,6 +1,5 @@
 import type { TaskGrade } from '$lib/types/task';
 import { isValidTaskGrade } from '$lib/utils/task_grade';
-import { browser } from '$app/environment';
 
 /**
  * Fetches the current user's vote grade for a given task.
@@ -88,14 +87,10 @@ function getBaseUrl(): string {
     return globalThis.location.origin;
   }
 
-  if (!browser) {
-    console.warn(
-      'getBaseUrl() called in SSR context where browser APIs are unavailable. ' +
-        'This should only be called from client-side code.',
-    );
-
-    return '';
-  }
+  console.warn(
+    'getBaseUrl() called in SSR context where browser APIs are unavailable. ' +
+      'This should only be called from client-side code.',
+  );
 
-  return 'http://localhost';
+  return '';
 }

src/features/votes/utils/median.ts

@@ -1,5 +1,6 @@
 import { TaskGrade } from '@prisma/client';
 import { getGradeOrder, taskGradeOrderInfinity } from '$lib/utils/task';
+import { MIN_VOTES_FOR_STATISTICS } from '$features/votes/constants/statistics';
 
 /** Maps grade order (1=Q11 … 17=D6) back to the corresponding TaskGrade enum value. */
 const ORDER_TO_TASK_GRADE: Map<number, TaskGrade> = new Map([
@@ -30,10 +31,14 @@ type GradeCounter = { grade: TaskGrade; count: number };
  * Returns `null` when the total vote count is below the minimum threshold.
  *
  * @param counters - Grade counters sorted by grade ascending.
- * @param minVotes - Minimum votes required to compute a median. Defaults to 3.
+ * @param minVotes - Minimum votes required to compute a median. Defaults to MIN_VOTES_FOR_STATISTICS.
+ *
  * @returns The median TaskGrade, or `null` if there are fewer than `minVotes` total votes.
  */
-export function computeMedianGrade(counters: GradeCounter[], minVotes = 3): TaskGrade | null {
+export function computeMedianGrade(
+  counters: GradeCounter[],
+  minVotes = MIN_VOTES_FOR_STATISTICS,
+): TaskGrade | null {
   const total = counters.reduce((sum, counter) => sum + counter.count, 0);
   if (total < minVotes) {
     return null;

src/test/utils/task_grade.test.ts src/lib/utils/task_grade.test.tssrc/test/utils/task_grade.test.ts renamed to src/lib/utils/task_grade.test.ts

@@ -9,6 +9,7 @@ export default defineConfig({
   },
   test: {
     include: [
+      'src/lib/**/*.test.ts', // shared utility tests
       'src/test/**/*.test.ts', // existing tests (phase transition)
       'src/features/**/*.test.ts', // feature co-location tests
     ],