feat: add hardcoded increment authority

[0xLeo-sqds]

Summary

Adds a hardcoded increment authority keypair that can call increment_account_index on any smart account without being a member. This allows Squads to provision vault indices on behalf of users.

Changes

• Added INCREMENT_AUTHORITY constant with testing/not(testing) feature flags
• Production key is Pubkey::default() (disabled) with a TODO: update before mainnet marker
• Testing key is DuFcCkArwSCTZ516uFbUcBYUM9TQdAo4xNDLWrzUnDGW with keypair at test-increment-authority.json
• Authority path bypasses membership/permission checks but still enforces FREE_ACCOUNT_MAX_INDEX cap
• Existing member-based increment path is unchanged

Test plan

• increment authority can increment without being a member — new test, passing
• All existing increment tests still pass (7 passing, 1 expected failure for max index)