- Create pyproject.toml with hatch build, httpx dependency
- Add src/logwell/__init__.py with __version__
- Add py.typed marker for type checking
- Add minimal README.md for hatchling build

Add types.py module with TypedDict definitions:
- LogLevel: Literal type for log severity levels
- LogEntry: Log entry with level, message, timestamp, metadata
- LogwellConfig: Client configuration with api_key, endpoint, callbacks
- IngestResponse: API response with accepted/rejected counts

Implements FR-1 (log methods), NFR-6 (type hints throughout).

- Add DEFAULT_CONFIG with batch_size, flush_interval, max_queue_size,
  max_retries, capture_source_location defaults
- Add API_KEY_REGEX pattern: lw_[32 alphanumeric chars]
- Add validate_api_key_format() function
- Add validate_config() that validates required fields, API key format,
  endpoint URL, numeric bounds, and merges with defaults

Implement HttpTransport class with:
- POST to /v1/ingest endpoint with Bearer auth header
- Exponential backoff with jitter (min(base * 2^attempt, 10s) + 30% jitter)
- Error classification by HTTP status:
  - 401 → UNAUTHORIZED (non-retryable)
  - 400 → VALIDATION_ERROR (non-retryable)
  - 429 → RATE_LIMITED (retryable)
  - 5xx → SERVER_ERROR (retryable)
- TransportConfig from LogwellConfig helper
- Async httpx client with lazy initialization

Requirements: FR-10, AC-5.2, AC-5.3, AC-5.4

Implements BatchQueue class with:
- Automatic flush on batch_size threshold
- Timer-based flush using threading.Timer
- Queue overflow protection (drops oldest)
- Re-queue on send failure
- Graceful shutdown with final flush
- Thread-safe operations with threading.Lock

Implement source_location module with:
- SourceLocation dataclass (frozen, slots)
- capture_source_location() function using inspect.stack()
- Supports skip_frames parameter to skip SDK internals
- Returns None when stack depth exceeded

Satisfies: FR-11, AC-6.1, AC-6.2, AC-6.3

Main entry point for the Python SDK that provides:
- All log level methods (debug, info, warn, error, fatal)
- Generic log() method for custom entries
- Automatic timestamp generation
- Source location capture when enabled
- Metadata merging with parent context
- Child logger creation with shared queue
- Graceful shutdown with flush

The client validates config on init, creates transport and queue,
and routes logs through the batch queue for efficient delivery.

Export Logwell, LogwellError, LogwellErrorCode, and all types from the
package's __init__.py for clean imports:

  from logwell import Logwell, LogwellError, LogwellErrorCode

Task: 1.9
Spec: python-sdk

POC E2E verification complete:
- Client instantiation works
- All log levels (debug, info, warn, error, fatal)
- Metadata attachment
- Child loggers with inherited metadata
- Flush and shutdown operations
- Type check (mypy) passes
- Error handling validated

Minor fix: removed slots=True from SourceLocation dataclass
for mypy compatibility.

- Verify thread safety already implemented in POC (threading.Lock on all ops)
- Fix lint issues: TC001 - move type imports to TYPE_CHECKING blocks
- Fix lint issues: UP035 - use collections.abc for Awaitable/Callable
- All queue/timer operations protected by self._lock

- Config errors: Include example usage and explain what's missing/wrong
- API key validation: Mask key in error, explain format requirements
- Endpoint validation: Show invalid URL, provide correct examples
- Numeric config: Show invalid value, explain purpose of each setting
- Transport errors: Include endpoint URL and timeout info
- HTTP errors: Explain what each status code means and what to do
- Queue overflow: Show max size, explain why it happened and fixes

Each error now answers: What failed? Why? How to fix it?

Verified mypy --strict passes on all source files.
Type hints were comprehensive from initial implementation.

Add pytest fixtures for SDK testing:
- Valid/invalid configuration fixtures
- Mock HTTP response fixtures (success, error, rate limit)
- Sample log entry fixtures
- Callback capture helpers for testing events
- Factory fixtures for creating test data

- Add tests/unit/__init__.py and tests/unit/test_config.py
- 78 tests covering validate_api_key_format and validate_config
- 100% code coverage of config.py module
- Test cases for:
  - Valid API keys (lowercase, uppercase, mixed case, numbers, hyphens, underscores)
  - Invalid API keys (wrong prefix, length, invalid chars, empty, None, wrong types)
  - Missing/empty required fields (api_key, endpoint)
  - Invalid endpoint URLs (no scheme, relative paths)
  - Numeric bounds (batch_size, flush_interval, max_queue_size, max_retries)
  - Default value merging
  - Optional fields (service, on_error, on_flush, capture_source_location)
  - Edge cases and validation order

47 tests covering:
- QueueConfig: construction, from_logwell_config
- BatchQueue: add, flush, size, shutdown methods
- Auto-flush on batch_size threshold
- Timer-based flush after flush_interval
- Queue overflow handling (drop oldest, on_error callback)
- Concurrent add/flush operations (thread safety)
- Edge cases (error recovery, entries during flush)

36 tests for source_location.py covering:
- SourceLocation dataclass: attributes, frozen/immutable, equality, repr
- capture_source_location: basic functionality, frame depths 0/1/2
- Invalid frame depths returning None
- Edge cases: lambda, list comprehension, nested calls
- Exception handling and graceful failure
- Integration-style tests for typical logging patterns

Requirements: AC-6.1, AC-6.2, AC-6.3

Add 67 comprehensive unit tests for client.py covering:
- Logwell construction with valid/invalid configs
- All log methods (debug, info, warn, error, fatal)
- Logging with metadata (none, empty, complex)
- Automatic UTC timestamp generation
- Service name handling (config, override, inheritance)
- queue_size property
- flush() method delegation and response handling
- shutdown() method (stops new logs, queue lifecycle)
- child() method (shares queue, inherits config, merges metadata)
- Nested children with accumulated metadata
- Source location capture when enabled
- _merge_metadata internal method
- Integration-style workflow tests

Add 22 integration tests using respx for HTTP mocking:
- Full flow tests (instantiate -> log -> flush -> verify)
- All log levels (debug, info, warn, error, fatal)
- Batching behavior and auto-flush on batch_size
- Retry on 5xx server errors and 429 rate limits
- Non-retryable error handling (401, 400)
- Shutdown behavior (flush remaining, idempotent, reject after)
- Child logger tests (same endpoint, metadata inheritance)
- Nested children metadata accumulation
- on_flush callback verification
- Source location capture when enabled
- Complex metadata serialization

- Fix E501 line too long errors in queue.py and transport.py
- All quality checks now pass:
  - mypy --strict: no issues
  - ruff check: all checks passed
  - pytest: 308 tests, 98% coverage

- Remove unused asyncio import from test_client.py
- Remove unused Any import from test_e2e.py

- Remove unused Callable and IngestResponse imports from test_client.py
- Remove unused Callable import from test_config.py
- Move IngestResponse and LogEntry to TYPE_CHECKING block in test_queue.py
- Remove empty TYPE_CHECKING block from test_source_location.py
- Run ruff format to fix import sorting