test(emit): add golden equivalence test for context_block() (#34)

[Sarcastic-Soul]

Follow-up to #34 / #23 as requested by @nossa-y.

What this changes

Adds a golden equivalence test (test_context_block_golden_equivalence) in tests/test_frames_emit.py:

1. Defines _context_block_legacy() as a reference implementation of the pre-optimization context_block() (which converted all frames via doc.to_dict(False) upfront).
2. Verifies that the optimized context_block() produces 100% byte-identical output against _context_block_legacy() across varied frame budgets (max_frames), duration filters (min_minutes), and text inclusion flags (include_text).

Checklist

• Output stays deterministic (no clock, network, randomness, or model calls)
• New behavior has a test; pytest -q passes
• New schema fields, if any, are measured - or live in the tier-2 inferred block with a confidence tag
• No typed text or page content is emitted by default