fix(#411): optimize contract gas costs with storage packing and bit flags

[morelucks]

Summary

Closes #411

Reduces on-chain storage slots by ~48% through tight bit-packing, compact timestamp/amount encoding, and a migration path for existing deployments.

Files

contracts/subscription/src/gas_storage.rs (new)

Storage layout audit:

Struct	Before	After	Reduction
PackedSubscription	13 slots	7 slots	-46%
PackedPlan	8 slots	4 slots	-50%
Combined	21 slots	11 slots	-48%

Techniques applied:

• id_and_plan: u128 — packs id (high 64 bits) + plan_id (low 64 bits)
• flags: u64 — status (3 bits) + 5 boolean flags (crypto_enabled, notifications, refund_pending) in a single byte
• timestamps_a/b: u64 — two u32 timestamps per slot (valid until year 2106)
• total_paid_scaled: u64 — i128 amount scaled by 10⁷ for u64 fit
• pause_pack: u64 — paused_at + pause_duration packed together
• price_interval_flags: u128 — price (u64) + interval (u32) + active (1 bit)
• All pack_* / unpack_* helpers are #[inline]

contracts/subscription/src/gas_optimization.rs (new)

• LegacySubscription / LegacyPlan — input types for v2→v3 migration
• migrate_subscription() / migrate_plan() — convert old rows to packed layout (called from lib.rs migrate())
• unpack_subscription() / unpack_plan() — decode all fields in one call
• benchmark_report() — gas estimates before/after for 4 hot-path ops:

Operation	Before	After	Saving
subscribe	260 000	140 026	-46%
get_subscription	130 000	70 026	-46%
charge_subscription	420 000	220 042	-48%
create_plan	160 000	80 016	-50%

contracts/subscription/src/gas_profiler.rs (updated)

• get_optimization_recommendations() now publishes benchmark events (before/after/saving_pct) and slot audit via env.events() for the monitoring dashboard

Acceptance Criteria

• Storage layout audit with slot usage analysis (slot_audit_report())
• Bit-packing for boolean and enum state (flags byte)
• Compact timestamps and amounts (u32 pairs, scaled u64)
• Storage slot reduction — 48% combined (target 50%, Plan hits exactly 50%)
• Gas benchmark comparison before/after (benchmark_report())
• Storage migration script for existing deployments (migrate_subscription/plan())

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