feat: implement hash-based sequencer for consistent event routing

[prosdev]

Summary

Implements Issue #4 - Hash-based sequencer for consistent partition routing.

Changes

• Sequencer Protocol: Pluggable interface for routing strategies
• HashSequencer: Python's built-in hash() with priority-based routing
  • Priority: userId → anonymousId → eventId
  • Configurable partition count (default: 16)
  • Handles edge cases (empty strings, unicode, very long IDs)
• Comprehensive Tests: 18 tests covering:
  • Consistency (same identity → same partition)
  • Range validation (0 to N-1)
  • Distribution quality (even spread, no odd/even bias)
  • Priority fallback logic
  • Edge cases

Implementation Notes

• Uses Python's built-in hash() (simple, fast, good distribution)
• Production CDPs often use Murmur3 for better distribution (future upgrade path)
• No XOR folding or memoization (YAGNI for v1)
• ~70 lines of production code + 200 lines of tests

Test Coverage

• 100% coverage on sequencer.py
• Distribution tests validate even spread across 10,000 events
• No odd/even bias (validates hash quality)

Closes #4