Phase 3c: Subscriptions — setNotifyValue, intent, re-arm

[itsniper]

Parent: #52

Requirements

• Subscribe/unsubscribe via platform setNotifyValue only—never write CCCD 0x2902 directly (FR-10.4.1).
• Confirm callback before active; sequence enable before notify-producing work (FR-10.4.2).
• Expose subscription state; don’t trust isNotifying alone (FR-10.4.3).
• Track subscription intent; re-arm after reconnect, restore, didModifyServices (FR-10.4.5).

Acceptance criteria

• Public subscribe API on Peripheral
• Intent survives reconnect/restore/modify paths (or explicit inactive reporting)
• Tests for enable confirm + re-arm