fix: use standard Substrate V4 format for unsigned extrinsics

[lcovar]

Summary

wasm-dot's unsigned extrinsic builder was encoding signed extensions (era, nonce, tip, etc.) in the extrinsic body before the call data:

compact(length) | 0x04 | era | nonce | tip | ... | call_data

Standard Substrate V4 unsigned format is:

compact(length) | 0x04 | call_data

Signed extensions belong only in the signing payload (computed by subxt-core's signer_payload()), not in the extrinsic body. The non-standard format caused any tool decoding the unsigned extrinsic to misread the extension bytes as the start of call data, producing invalid pallet/method indices (e.g. [43, 127] instead of [10, 3] for balances.transferKeepAlive).

This didn't affect signed transactions (subxt rebuilds the full extrinsic correctly during signing) or broadcast, but broke offline verification and any external decoder.

Changes

• builder/mod.rs: simplified build_unsigned_extrinsic to just 0x04 | call_data, removed signed extension encoding logic (era/nonce/tip/CheckMetadataHash/ChargeAssetTxPayment). Era/nonce/tip are now set on the Transaction struct from the build context instead.
• transaction.rs: updated unsigned extrinsic parser to read call data immediately after version byte (no extension parsing for unsigned).
• Removed is_empty_type and encode_zero_value helpers from builder (only needed for extension encoding, still in transaction.rs for signed parsing).

Testing

All 26 existing tests pass. The fix is backwards-compatible for consumers: signablePayload(), addSignature(), and toBroadcastFormat() continue to work correctly since they use subxt-core APIs that handle extensions internally.