v4: add ordered random amount allocator

[Phloraxx]

Scope

Second stacked PayGate v4 implementation slice. Depends on PR #43.

Behavior

• for a ₹N request, choose cryptographically at random among free ₹N.01…₹N.99
• only when the entire base bucket is unavailable, choose randomly from ₹(N+1).01…₹(N+1).99
• never overflow while even one base-bucket candidate is free
• .00 is never generated
• release due hard reservations inside the same immediate SQLite transaction
• soft recent-use avoidance prefers older/unseen values but cannot create fake exhaustion or force overflow
• return a clean capacity error only when both configured buckets are unavailable

Tests

Covers random base selection, one-free-slot no-overflow, full-base overflow, recent-use fallback, old-value preference, due-release behavior, full two-bucket exhaustion, invalid requested paise, race tests and vet.

Safety

No production route/runtime wiring. The allocator requires storage.ImmediateTx, so selection cannot accidentally happen outside the payment write transaction.