Implements `dreamer-design-spec.md` lines 27-74: observe_corpus() returns a
signal vector (new_chunks delta, new_journal_entries, recent_questions over
14-day window, days_since_dream, underprocessed_count derived from the new
consolidation cursor); select_mode() returns one of {nrem, early-rem,
late-rem, lucid} or None per the spec's rules. The None return is the spec's
canonical answer to the repetition problem (line 67) — "dreamer goes quiet
rather than manufacturing novelty."
Standalone for now. Not wired into dream_pipeline yet — that happens in the
retrieve() refactor (task #46). dream.py is unchanged in this commit.
Grounded sources cited in module docstring: Friston Active Inference, sleep
research (Stickgold/Walker/Diekelberg & Born), sharp-wave ripples (Buzsáki).
All three appear in BirdAI-Bibliography.md.
Migration prerequisite (already shipped in the prior commit): consolidation
cursor columns last_consolidated_at + consolidation_count added to
embeddings. Backfill from dream-manifest history is task #49.
