feat: add max_total_supply_shares hard cap independent of bps

[Hunter-baddie]

Summary of Changes

1. Updated test_close_period.rs:

• Added linearity test : close_period_cpu_grows_linearly_with_holders that parameterizes over holder counts [1,10,100,1000], measures CPU instruction usage, and asserts R² > 0.98
• Added constant cost test : close_period_zero_holders_has_constant_cost that verifies closing a period with zero holders stays within a constant CPU budget
• Implemented helper functions :
  • measure_cpu_for_n_holders : to measure CPU delta for a specific number of holders
  • r_squared : calculates coefficient of determination for linear fit

2. Created Documentation:

• CLOSE_PERIOD_GAS_TEST_SUMMARY.md : a markdown file documenting the test cases, linearity check, key assumptions, and security notes, following the style of the multisig gas test summary
  The implementation maintains backward compatibility, follows the existing code style, and ensures that any future changes to close_period that introduce non-linear (e.g., O(n²)) performance will be caught by the tests!
  close Add issuance cap with explicit max_total_supply_shares decoupled from supply_cap bps #474
  close Add gas-bound test for atomic_close_period asserting linear-in-holders cost bound #480

[drips-wave]

@Hunter-baddie Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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