Add shared PLUME signature test vectors

[JustABiologist]

Closes #24

Summary

• add a shared test-vectors/plume-signatures.json fixture for a deterministic low-entropy V1/V2 PLUME signing case
• have RustCrypto, Arkworks, and Circom JS tests load the same vector instead of repeating constants
• keep the existing JS signal expectations while deriving them from the shared fixture

Verification

• corepack pnpm@8.15.9 run types in circuits/circom/test/javascript
• corepack pnpm@8.15.9 test -- signals.test.ts in circuits/circom/test/javascript
• corepack pnpm@8.15.9 exec prettier -c test/consts.ts test/signals.test.ts --config ../../../../.prettierrc --ignore-path ../../../../.prettierignore
• git diff --check

Not run locally: Rust tests because this environment does not have cargo; full top-level Circom install because frozen install is blocked by the current lockfile metadata and CI already uses --no-frozen-lockfile.

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