fix: add NULL hash guards to prevent v5.3.3 regression

Fixed critical bug where CommitObject.walk() didn't guard against NULL parent
hash, causing "Blob metadata not found: 0000...0000" error when calling
getHistory() on fresh databases.

This is a SYSTEMATIC fix, not just a patch:

Infrastructure:
- src/storage/cow/constants.ts: NEW - NULL_HASH constant and utilities
- Prevents hardcoding errors
- Provides isNullHash() for semantic checks

Bug Fixes:
- src/storage/cow/CommitObject.ts: Guard NULL parent in walk()
- src/storage/cow/BlobStorage.ts: Defensive check rejects NULL hash reads
- src/storage/baseStorage.ts: Use NULL_HASH constant (not hardcoded)
- src/brainy.ts: Use NULL_HASH constant (not hardcoded)

Testing:
- tests/integration/initial-commit-null-parent.test.ts: 5 regression tests
- All 17 tests pass (5 new + 12 existing COW tests)
- Zero regressions

This fix addresses the root cause of 4 consecutive bugs (v5.3.0-v5.3.3):
missing defensive programming for sentinel values in COW storage.

Fixes: Workshop team bug report (v5.3.3 regression)
Tests: 17/17 pass (5 new regression + 12 existing COW)
Build: SUCCESSFUL (zero errors)

tests/integration/initial-commit-null-parent.test.ts

@@ -0,0 +1,143 @@
+/**
+ * Regression test for v5.3.3 NULL parent bug
+ *
+ * BUG: CommitObject.walk() didn't guard against NULL parent hash, causing
+ * "Blob metadata not found: 0000...0000" error when calling getHistory()
+ * on a fresh database with only the initial commit.
+ *
+ * ROOT CAUSE: Initial commit has parent = null or NULL_HASH ('0000...0000'),
+ * but while(currentHash) treated the string as truthy and tried to read it.
+ *
+ * FIXED IN: v5.3.4
+ * - Added isNullHash() guard in CommitObject.walk()
+ * - Added defensive check in BlobStorage.read()
+ * - Created NULL_HASH constant to prevent hardcoding
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import { Brainy } from '../../src/brainy.js'
+import { NULL_HASH } from '../../src/storage/cow/constants.js'
+import * as fs from 'fs'
+
+const TEST_DATA_PATH = './test-null-parent-data'
+
+describe('Initial Commit NULL Parent Bug (v5.3.3 regression)', () => {
+  let brain: Brainy
+
+  beforeEach(async () => {
+    // Clean up test data
+    if (fs.existsSync(TEST_DATA_PATH)) {
+      fs.rmSync(TEST_DATA_PATH, { recursive: true, force: true })
+    }
+
+    brain = new Brainy({
+      storage: {
+        type: 'filesystem',
+        path: TEST_DATA_PATH,
+        branch: 'main',
+        enableCompression: true
+      },
+      disableAutoRebuild: true,
+      silent: true
+    })
+
+    await brain.init()
+  })
+
+  afterEach(() => {
+    if (fs.existsSync(TEST_DATA_PATH)) {
+      fs.rmSync(TEST_DATA_PATH, { recursive: true, force: true })
+    }
+  })
+
+  it('should handle getHistory() on fresh database with only initial commit', async () => {
+    // Fresh brain has only the initial commit (created during init)
+    // This was throwing "Blob metadata not found: 0000...0000" in v5.3.3
+
+    // Get history - should NOT throw
+    const history = await brain.getHistory({ limit: 10 })
+
+    // Should return the initial commit
+    expect(history).toBeDefined()
+    expect(Array.isArray(history)).toBe(true)
+    expect(history.length).toBe(1)
+
+    // Initial commit should have specific properties
+    const initialCommit = history[0]
+    expect(initialCommit.message).toBe('Initial commit')
+    expect(initialCommit.author).toBe('system')
+    // Parent can be null or undefined for initial commit
+    expect(initialCommit.parent == null).toBe(true)
+  })
+
+  it('should stop walk at initial commit (parent = null)', async () => {
+    // Note: brain.commit() creates "Snapshot commit" messages by default
+    // Create 2 commits
+    await brain.add({ data: 'First entity', type: 'concept' })
+    await brain.commit('First user commit')
+
+    await brain.add({ data: 'Second entity', type: 'concept' })
+    await brain.commit('Second user commit')
+
+    // Get full history
+    const history = await brain.getHistory({ limit: 100 })
+
+    // Should have at least 3 commits (2 user + 1 initial)
+    expect(history.length).toBeGreaterThanOrEqual(3)
+
+    // Last commit should be initial commit with no parent
+    const initialCommit = history[history.length - 1]
+    expect(initialCommit.message).toBe('Initial commit')
+    // Parent can be null or undefined for initial commit
+    expect(initialCommit.parent == null).toBe(true)
+  })
+
+  it('should not attempt to read NULL hash from BlobStorage', async () => {
+    // Direct test: attempting to read NULL_HASH should throw clear error
+    const blobStorage = (brain as any).storage.blobStorage
+
+    // Should throw clear error message
+    await expect(
+      blobStorage.read(NULL_HASH)
+    ).rejects.toThrow(/sentinel value/)
+
+    // Error message should mention what NULL_HASH is for
+    await expect(
+      blobStorage.read(NULL_HASH)
+    ).rejects.toThrow(/no parent/)
+  })
+
+  it('should handle multiple calls to getHistory() consistently', async () => {
+    // Ensure caching doesn't break NULL hash handling
+
+    // First call
+    const history1 = await brain.getHistory({ limit: 10 })
+    expect(history1.length).toBe(1)
+
+    // Add a commit
+    await brain.add({ data: 'Test entity', type: 'concept' })
+    await brain.commit('Test commit')
+
+    // Second call (should now have 2 commits)
+    const history2 = await brain.getHistory({ limit: 10 })
+    expect(history2.length).toBe(2)
+
+    // Third call (should still have 2 commits)
+    const history3 = await brain.getHistory({ limit: 10 })
+    expect(history3.length).toBe(2)
+
+    // All should end with initial commit
+    expect(history1[history1.length - 1].message).toBe('Initial commit')
+    expect(history2[history2.length - 1].message).toBe('Initial commit')
+    expect(history3[history3.length - 1].message).toBe('Initial commit')
+  })
+
+  it('should handle walking with maxDepth on initial commit', async () => {
+    // Edge case: maxDepth = 1 on fresh database
+
+    const history = await brain.getHistory({ limit: 1 })
+
+    expect(history.length).toBe(1)
+    expect(history[0].message).toBe('Initial commit')
+  })
+})