feat: optional RAM promotion for octad acceleration

Tiered storage with per-modality RAM promotion/demotion.
DISABLED by default. EXPLICIT opt-in only.

Constraints:
- Max 2 octads promoted simultaneously (crash safety)
- Max 5 minutes in RAM before forced demotion
- Only if estimated speedup >= 2x (MIN_SPEEDUP_FACTOR)
- RAM budget enforced (default 1GB)
- All data ALWAYS on disk via WAL — RAM is read cache only
- WAL on disk is source of truth, never tmpfs
- On crash: WAL replays, RAM overlay lost, zero data loss

API:
- PromotionManager::new() — disabled by default
- enable(budget) / disable()
- decide(modalities, data_size, disk_time, ram_time) → Promote/Skip/Blocked
- promote(modality, size) / demote(modality) / demote_all()
- enforce_timeouts() — auto-demotes expired promotions
- history() — full promotion/demotion event log

8 tests: disabled default, enable+promote, max-2 limit, demote frees slot,
RAM budget enforcement, speedup threshold, demote_all, history tracking.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

verisimdb/rust-core/verisim-octad/src/lib.rs

@@ -38,6 +38,10 @@ pub use store::{OctadSnapshot, InMemoryOctadStore};
 pub mod query_octad;
 pub use query_octad::{QueryOctadBuilder, QueryExecution};
 
+// Optional RAM promotion for acceleration (disabled by default)
+pub mod ram_promotion;
+pub use ram_promotion::{PromotionManager, Modality, PromotionDecision, PromotionEvent};
+
 // ACID transaction manager for cross-modality atomicity
 pub mod transaction;
 pub use transaction::{IsolationLevel, LockType, TransactionManager, TransactionError, TransactionState};