Content Hash Cache Pattern

# Content-Hash File Cache Pattern

Cache expensive file processing results (PDF parsing, text extraction, image analysis) using SHA-256 content hashes as cache keys. Unlike path-based caching, this approach survives file moves/renames and auto-invalidates when content changes.

## When to Activate

- Building file processing pipelines (PDF, images, text extraction)
- Processing cost is high and same files are processed repeatedly
- Need a `--cache/--no-cache` CLI option
- Want to add caching to existing pure functions without modifying them

## Core Pattern

### 1. Content-Hash Based Cache Key

Use file content (not path) as the cache key:

```python
import hashlib
from pathlib import Path

_HASH_CHUNK_SIZE = 65536  # 64KB chunks for large files

def compute_file_hash(path: Path) -> str:
    """SHA-256 of file contents (chunked for large files)."""
    if not path.is_file():
        raise FileNotFoundError(f"File not found: {path}")
    sha256 = hashlib.sha256()
    with open(path, "rb") as f:
        while True:
            chunk = f.read(_HASH_CHUNK_SIZE)
            if not chunk:
                break
            sha256.update(chunk)
    return sha256.hexdigest()
```

**Why content hash?** File rename/move = cache hit. Content change = automatic invalidation. No index file needed.

### 2. Frozen Dataclass for Cache Entry

```python
from dataclasses import dataclass

@dataclass(frozen=True, slots=True)
class CacheEntry:
    file_hash: str
    source_path: str
    document: ExtractedDocument  # The cached result
```

### 3. File-Based Cache Storage

Each cache entry is stored as `{hash}.json` — O(1) lookup by hash, no index file required.

```python
import json
from typing import Any

def write_cache(cache_dir: Path, entry: CacheEntry) -> None:
    cache_dir.mkdir(parents=True, exist_ok=True)
    cache_file = cache_dir / f"{entry.file_hash}.json"
    data = serialize_entry(entry)
    cache_file.write_text(json.dumps(data, ensure_ascii=False), encoding="utf-8")

def read_cache(cache_dir: Path, file_hash: str) -> CacheEntry | None:
    cache_file = cache_dir / f"{file_hash}.json"
    if not cache_file.is_file():
        return None
    try:
        raw = cache_file.read_text(encoding="utf-8")
        data = json.loads(raw)
        return deserialize_entry(data)
    except (json.JSONDecodeError, ValueError, KeyError):
        return None  # Treat corruption as cache miss
```

### 4. Service Layer Wrapper (SRP)

Keep the processing function pure. Add caching as a separate service layer.

```python
def extract_with_cache(
    file_path: Path,
    *,
    cache_enabled: bool = True,
    cache_dir: Path = Path(".cache"),
) -> ExtractedDocument:
    """Service layer: cache check -> extraction -> cache write."""
    if not cache_enabled:
        return extract_text(file_path)  # Pure function, no cache knowledge

    file_hash = compute_file_hash(file_path)

    # Check cache
    cached = read_cache(cache_dir, file_hash)
    if cached is not None:
        logger.info("Cache hit: %s (hash=%s)", file_path.name, file_hash[:12])
        return cached.document

    # Cache miss -> extract -> store
    logger.info("Cache miss: %s (hash=%s)", file_path.name, file_hash[:12])
    doc = extract_text(file_path)
    entry = CacheEntry(file_hash=file_hash, source_path=str(file_path), document=doc)
    write_cache(cache_dir, entry)
    return doc
```

## Key Design Decisions

| Decision | Rationale |
|----------|-----------|
| SHA-256 content hash | Path-independent, auto-invalidates on content change |
| `{hash}.json` file naming | O(1) lookup, no index file needed |
| Service layer wrapper | SRP: extraction stays pure, cache is a separate concern |
| Manual JSON serialization | Full control over frozen dataclass serialization |
| Corruption retu