feat: Implement Development Grants Fund contract (SOV-L0-2.3)

- Implement core DevelopmentGrants contract with fee collection and governance-controlled disbursements
- Add comprehensive type definitions: ProposalId, Amount, Recipient, Disbursement
- Enforce all 10 invariant sections:
  * Fee routing validation (F1, F2): Passive receiver with amount > 0 check
  * Account balance conservation (A1, A2, A3): balance = fees - disbursements, append-only ledger
  * Governance authority (G1, G2, G3): Governance-only, proposal binding, replay protection
  * State mutation (S1, S3): Deterministic, no reentrancy risk
  * Boundary conditions: Overflow/underflow checks, edge cases
  * Year-5 target (T2): Correctly non-enforceable at contract level
  * Upgrade compatibility (U1): Serializable state for persistence
  * Read-only registry (U2): No external contract calls
  * Failure modes: All errors halt execution explicitly
  * Security boundary: No arbitrary withdrawals, governance-only control
- Add 13 comprehensive unit tests covering all invariant classes
- All tests passing (100% pass rate)

Author: umwelt