feat: harden AnonVote contract with validation, error taxonomy, admin… - #95

Hardening AnonVote Soroban Contract
Input validation

• All write paths (record_ballot, record_token, record_vote, record_result) validate hashes via shared validate_hex_hash — rejects non-64-char strings, uppercase, and non-hex characters
• initialize rejects zero address (GAAA...A)
• rotate_admin rejects zero address and self-rotation
  State guards
• Explicit DataKey::Initialized flag prevents re-initialization (was implicit via Admin presence)
• record_token / record_vote / record_result check ballot existence → BallotNotFound
• record_token / record_vote / record_result check finalisation → BallotAlreadyFinalised
• record_ballot rejects duplicate hashes → BallotAlreadyExists
  Error taxonomy (errors.rs)
  10 stable #[repr(u32)] discriminants (must never change post-deployment):
• AlreadyInitialized(1), Unauthorized(2), BallotAlreadyExists(3), BallotNotFound(4), BallotAlreadyFinalised(5), InvalidBallotIdHash(6), InvalidResultHash(7), InvalidAdminAddress(8), CounterOverflow(9), BallotExpired(10)
  Overflow detection
• u32::MAX guard on token/vote counters returns CounterOverflow + emits ("counter", "ovrflw") event
  Admin rotation
• rotate_admin stores rotation history as Vec (old_admin, new_admin, timestamp)
• New get_admin_history view function
  Breaking changes from previous behaviour
• Old idempotent re-record removed: record_ballot with existing hash always fails (BallotAlreadyExists)
• Old idempotent re-record removed: record_result with any hash on a finalised ballot fails (BallotAlreadyFinalised)
• Event topics renamed from ("audit", "blt_crtd") → ("ballot", "regstrd") etc.
  Testing (29 cases)
  initialize guards, hash validation (length/uppercase/non-hex), duplicate detection, ballot-not-found, finalisation guards, overflow at u32::MAX (with non-panicking isolation), admin rotation (success, same-address, zero-address, unauthorized, history growth), result hash validation, is_consistent for active/inconsistent/nonexistent ballots.
  Stability
  STABILITY.md documents that error discriminants, storage keys, and event topics are immutable post-deployment.

closes #90