Testing: Add test for quadratic voting cost calculation

[Mosas2000]

Summary

The quadratic voting mechanism (cost = weight squared) is the project's core innovation and should have thorough dedicated tests. These tests should verify:

1. vote-weight of 1 costs 1 STX
2. vote-weight of 5 costs 25 STX
3. vote-weight of 10 costs 100 STX
4. vote-weight of 0 is handled (should be rejected)
5. Large vote weights do not cause overflow
6. Cost is properly deducted from stake (once the deduction bug is fixed)
7. Insufficient stake for vote cost is properly rejected

Impact

High - Core mechanism correctness must be verified.

[Mosas2000]

Resolved in PR #198. The sprintfund-core.test.ts includes dedicated quadratic voting cost calculation tests:

• Test for weight 1 (cost 1)
• Test for weight 10 (cost 100)
• Test for weight 100 (cost 10000)
• Test for rejection when cost exceeds stake (weight 10001 costs 100020001 > 100000000 stake)