fix: update hash format validation to -01 spec (plain base64url, no prefix)

Go ValidateHashFormat was still validating the old -00 format
(algorithm:base64url with sha-256/sha-384/sha-512 prefix). Updated to
validate plain base64url without prefix per -01 spec and RFC 9449.
Python was already updated but uncommitted. Both refimpls now match.

refimpl/IMPROVEMENTS.md

@@ -50,7 +50,7 @@ Suggestions that could make the implementations more robust, spec-strict, or pro
 ## 5. **Nice-to-have** ✅
 
 - **inp_hash / out_hash format**  
-  **Done.** Optional check in create and verify: `algorithm:base64url` with algorithm in allowlist (sha-256, sha-384, sha-512). Helpers: `ValidateHashFormat` / `validate_hash_format`.
+  **Done.** Optional check in create and verify: plain base64url without algorithm prefix, per -01 spec and RFC 9449. Helpers: `ValidateHashFormat` / `validate_hash_format`.
 
 - **Constant-time comparison**  
   **Done.** **Go:** `crypto/subtle.ConstantTimeCompare` for `typ` in verify. **Python:** `hmac.compare_digest` for `typ`.
@@ -72,4 +72,4 @@ The refimpl was built against draft-nennemann-wimse-ect-00. The -01 draft introd
 - **Update `MaxParLength` naming**: ✅ **Done.** Renamed to `MaxPredLength` / `max_pred_length` everywhere.
 - **Add L1 support**: The -01 draft introduces unsigned JSON ECTs (Level 1). The refimpl currently only supports L2 (signed JWS).
 - **Add L3 support**: The -01 draft introduces audit ledger requirements for Level 3. The existing in-memory ledger needs hash chain and receipt support.
-- **Update hash format**: The -01 draft specifies SHA-256 base64url without algorithm prefix (no `sha-256:` prefix), consistent with RFC 9449.
+- **Update hash format**: ✅ **Done.** Both Go and Python validate plain base64url without algorithm prefix, consistent with -01 spec and RFC 9449.