Feat/storage cpu micro optimizations

[Kingsman-99]

Fix #25

Changes

• policy.rs — removed dead compile-time branch (QUOTE_TTL_LEDGERS == 0)
• premium.rs — removed unchecked compute_premium (overflow risk); optimized
  build_line_items to compute each factor once instead of twice
• storage.rs — added comments documenting write counts and storage tier rationale
• PERFORMANCE.md — before/after for all 6 optimization areas including integer
  width audit and storage tier audit
• .gitignore — updated with all necessary exclusions

Closes #25

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