feat(session): Phase 1.5 — Session rotation 4h epochs canonical v0.0.7

Origin Chain runtime canonical ahora emite SessionEvent::NewSession cada 4
horas exactas per CHN-1 ratified. Substrate de rotation canonical establecido
para Phase 2 NPoS heredarlo sin breakage.

## Pallet añadido (per SBS-4)
- Index 6 Session (pallet_session 40.0.1) — validator session rotation per
  CHN-1 4h epochs. Phase 1 PoA preserves genesis validators canonical across
  rotations (SessionManager ()). Phase 2 NPoS introducira pallet_staking como
  SessionManager para rotation real per nominator stake + Pi 5 hardware tier
  weight.

## Configs canonical pallet_session
- SessionPeriod = BLOCK_HASH_COUNT (2400 bloques × 6s = 14400s = 4h exactas)
- SessionOffset = 0 (sesiones start at block 0)
- ValidatorId = ChainAccountId
- ValidatorIdOf = AccountIdIsValidatorId (custom identity converter Phase 1 PoA)
- ShouldEndSession/NextSessionRotation = PeriodicSessions<SessionPeriod, SessionOffset>
- SessionManager = () (preserves genesis Phase 1 PoA)
- SessionHandler = <opaque::SessionKeys as OpaqueKeys>::KeyTypeIdProviders
- Keys = opaque::SessionKeys (aura sr25519 + grandpa Ed25519)
- DisablingStrategy = () (Phase 2 NPoS introduce real slashing-based disable)

## Custom converter canonical
AccountIdIsValidatorId struct nueva — identity mapping AccountId → ValidatorId
Phase 1 PoA trivial. Phase 2 NPoS reemplazara con pallet_staking::StashOf<Self>
para real Stash/Controller separation canonical.

## Filosofía canonical
SessionPeriod = 4h es la frontera temporal canonical: cada 4h el chain
reflexiona on-chain quienes son los validators activos. Esta cadencia
institucional canonical es la minima necesaria para que Phase 2 NPoS pueda
evolucionar el set sin re-launching genesis. David canon 2026-05-14
materializado temporalmente.

## Build + tests
- cargo check 1.63s warm cache (pallet_session incremental)
- cargo build --release 7.70s warm cache clean
- ~57 tests passing (count preserved — session tests Phase 1.7 chain_spec)

## Decision empirica
pallet_session::Config::DisablingStrategy es nuevo stable2512 canonical (no
estaba en versiones anteriores). Phase 1 PoA: () preserva behavior original.
Phase 2 NPoS: UpToLimitWithReEnablingDisablingStrategy per CHN-3.

## Phase 1 progress
1.1 ✅ + 1.2 ✅ + 1.3 ✅ + 1.4 ✅ + 1.5 ✅ (esta). Next: 1.6 pallet_identity
Ed25519 DID + origin-chain-node real service.

7 pallets composed canonical:
- 0 System + 1 Timestamp + 2 Balances + 3 TransactionPayment
- 4 Aura + 5 Grandpa + 6 Session ← NEW
- Opaque SessionKeys consumed by pallet_session::Config::Keys ← NEW WIRE

Refs: CHARON_ROADMAP_V1 Phase 1.5, CHN-1 4h epoch rotation canonical, SBS-4
extended, arkhe-origin-chain commit pending

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/origin-chain-runtime/src/lib.rs

@@ -217,6 +217,13 @@ mod runtime {
     /// (Aura ~6s) de finality cadence (canonical ~6-12s combined).
     #[runtime::pallet_index(5)]
     pub type Grandpa = pallet_grandpa::Pallet<Runtime>;
+
+    /// Session pallet canonical (validator rotation per CHN-1 4h epochs).
+    /// Phase 1 PoA: preserves genesis validators canonical across rotations.
+    /// Phase 2 NPoS: pallet_staking will be SessionManager canonical para real
+    /// rotation per nominator stake + Pi 5 hardware tier weight.
+    #[runtime::pallet_index(6)]
+    pub type Session = pallet_session::Pallet<Runtime>;
 }
 
 #[cfg(test)]