fix(multichain): reject EVM chain submissions with explicit error and eliminate fabricated transaction hashes

[mopxam-art]

Why

src/lib/multichain.ts synthesized artificial hexadecimal strings and formatted them as transaction hashes (0x${chain.id}_${projectId}_... and 0x${Buffer.from(...)}) for EVM chains (Ethereum and Polygon). These fake hashes were presented as successful cross-chain submissions in API responses and internal broadcast results, even though no EVM contract call was executed.

What

• Removed synthesized transaction hash paths in submitToChain (src/lib/multichain.ts).
• Explicitly return unconfigured error (EVM chain submission not configured for ...: missing rpcUrl or contractAddress) when configuration is absent, and not-implemented error (EVM chain submission is not implemented for ...) when configured, leaving txHash empty ("") and success: false.
• Updated test assertions in src/__tests__/multichain.test.ts to verify that no fabricated hash is ever emitted.

Verified

• Added comprehensive unit tests in src/__tests__/multichain-no-fake-hashes.test.ts (105 passing, 149 across multichain suites) proving zero synthetic hashes are emitted across all project IDs, score boundaries, and target configurations.
• All 123 test suites (1,524 tests) pass (npm test).
• Typecheck (npx tsc --noEmit) and linter pass with 0 errors.
• Production build (npm run build) succeeded.

Closes #921

Payout coordinate:
USDC (EVM / Base / Arbitrum / ERC20): 0x387c064ccf729a4846b62e95dc6b70e4d4c41e73