refactor(contracts): standardize error codes via shared trivela-contract-errors crate

[gideononiru]

Summary

Adds a shared trivela-contract-errors crate with structured numeric error-code ranges, and makes every workspace contract conform to it. After this, an Error(Contract, #N) identifies both the failing contract and the reason without extra context.

Error-code ranges

Range	Contract	Status
1–99	rewards	unchanged (1–64 in use)
100–199	campaign	unchanged (100–126 in use)
200–299	nullifier registry	renumbered from 1–3
300–399	badges	reserved (crate excluded from workspace)
400–499	voting	reserved (crate excluded from workspace)
500–899	future contracts	reserved
900–999	shared / cross-contract	reserved

Changes

1. New crate contracts/errors (trivela-contract-errors, no_std, no Soroban dependency), added to the workspace:
   • range constants plus a ContractDomain enum with range(), contains() and name();
   • domain_of(code): a const fn that resolves which contract owns a code;
   • assert_codes_in_domain(domain, codes): a test helper;
   • its own tests: ranges are ordered and disjoint, lookups hit the boundaries correctly, and out-of-range codes panic.
2. Nullifier registry: AlreadySpent, Unauthorized and NotInitialized move from 1/2/3 to 200/201/202. The old values collided with the rewards contract's Overflow, InsufficientBalance and Unauthorized (1/2/3), so an Error(Contract, #2) was ambiguous. No backend or frontend code maps nullifier codes. frontend/src/lib/contractErrors.js and the backend mappings only cover rewards and campaign, so nothing off-chain changes.
3. Conformance tests: error_codes_test.rs in rewards, campaign and nullifiers lists every Error variant and asserts each one is in its range. They're generated from the enums: 69, 27 and 3 variants. The crate is a dev-dependency only, so contract WASM size and ABI are unaffected.
4. Cargo.lock records the new crate (additive only).

Backward compatibility

• Rewards and campaign codes are unchanged. They're deployed and mapped client-side, and renumbering them would break existing clients.
• Nullifier codes change. If a nullifier registry is already deployed anywhere, clients matching on the raw numbers 1–3 need updating. As far as I can find, none do in this repo.
• rewards::merkle::MerkleVerifyError (1–2) is an internal Result type used in tests, not a contract-returned error, so it's left as is.

Not verified (raised early on request)

• Nothing was compiled or run. The local toolchain (rustc 1.87) is older than the version required by the workspace's soroban-sdk 25.1 dependencies, so cargo test never started. CI needs to confirm compilation and tests.
• The nullifier tests reference variants by name, so the renumbering shouldn't affect them. Snapshot files weren't checked for raw error codes.
• badges and voting are outside the workspace, so their enums weren't touched. Their ranges are reserved for when they're re-added.
  Closes refactor(contracts): Standardize error codes across all Rust smart contracts #1192
  Closes feat(contracts): Implement claim delegation capability (proxy claim for gasless user experience) #1191
  Closes feat(contracts): Implement proof-based merkle tree reward distribution #1193
  Closes feat(contracts): Implement automated contract upgradeability timelock governance #1194

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