Add contract storage usage estimator

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Creators with many invoices may want to estimate their share of contract storage rent burden, useful for cost forecasting before creating large batches.

Technical Context

New src/storageUsageEstimator.ts. Estimates per-invoice storage footprint (core + ext + ext2 entries) based on known field sizes from src/types.ts, multiplied by invoice count.

Acceptance Criteria

• estimateStorageFootprint(invoiceCount, optionsUsed) returns an estimated byte count and rough rent-cost estimate
• Accounts for optional features increasing footprint (tranches, co-signers, split rules) via a documented per-feature byte estimate
• Pure function, no network calls
• Tests: baseline estimate; estimate increases correctly when optional features are toggled on

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