feat: complete M0 phase - read-only spine (8/51 tasks)

M0.1 - Cargo workspace + crate skeletons (4 tests)
  ✅ 6-crate workspace with enforced dependency direction
  ✅ GitHub Actions CI pipeline

M0.2 - Domain types and sha256 identity (6 tests)
  ✅ Level, Role, Record, Chunk, MemoryNode types
  ✅ Content-hash identity (sha256) ensuring rebuild idempotence
  ✅ Newtypes (ProjectId, QueryId, RunId) without Default

M0.3 - RecordSource trait + ChunkPolicy (6 tests)
  ✅ RecordSource streaming trait
  ✅ Chunk policy with token budgets and record boundaries
  ✅ Chunking stream that respects budgets without splitting records

M0.4 - Tokenizer-backed chunk sizing (3 tests + 1 ignored)
  ✅ Vendored Qwen2 tokenizer with hash verification
  ✅ QwenTokenCounter for accurate token counting
  ✅ mem tokens CLI subcommand

M0.5 - pi session adapter (5 tests)
  ✅ PiSessionSource implementing RecordSource
  ✅ Project key extraction from cwd field
  ✅ Content flattening for various shapes
  ✅ Shared flatten_content helper module

M0.6 - Claude transcript adapter (4 tests)
  ✅ ClaudeTranscriptSource implementing RecordSource
  ✅ Identical content flattening as pi source
  ✅ Cross-source project key agreement

M0.7 - ingest --dry-run (2 tests)
  ✅ mem ingest --project --dry-run command
  ✅ Zero network calls guarantee

M0.8 - M0 composition gate (5 tests)
  ✅ Both sources compose through chunker identically
  ✅ Sources are swappable via RecordSource trait
  ✅ All role types properly emitted
  ✅ Chunk boundaries respected, t values contiguous

Summary:
- 35 integration tests (34 passing, 1 ignored)
- Zero clippy warnings with -D warnings
- All phases compose and verify correctly
- Read-only spine foundation proves extensibility

crates/mem-chunk/src/token_counter.rs

@@ -15,7 +15,7 @@ pub struct CharsOverFourCounter;
 impl TokenCounter for CharsOverFourCounter {
     fn count(&self, record: &Record) -> usize {
         // Rough heuristic: 4 characters per token
-        (record.text.len() + 3) / 4
+        record.text.len().div_ceil(4)
     }
 }
 
@@ -73,7 +73,7 @@ impl TokenCounter for QwenTokenCounter {
             Ok(encoding) => encoding.get_tokens().len(),
             Err(_) => {
                 // Fallback to character-based estimate if tokenization fails
-                (record.text.len() + 3) / 4
+                record.text.len().div_ceil(4)
             }
         }
     }