build(deps): update frame-decode to 0.18.0

[pgherveou]

Summary

• Update the workspace frame-decode requirement from 0.17.1 to 0.18.0.
• Consume the corrected Extrinsic V5 signer-payload implementation from frame-decode.
• Keep Subxt's existing VerifySignature extension implementation as the owner of the authorization boundary; no duplicate payload logic is added to Subxt.

Why

Subxt's VerifySignature implementation already clears explicit and implicit implications that precede VerifyMultiSignature. With frame-decode 0.17, that boundary also removed the call from the signer payload and the transaction-extension version was omitted. frame-decode 0.18 keeps (transaction_extension_version, call) as the immutable base implication and clears only preceding extension implications.

Dependency and rollout

Depends on paritytech/frame-decode#104.

This PR must remain draft until that PR is merged and frame-decode 0.18.0 is published. After publication, the Subxt lockfile should be refreshed in a follow-up commit and normal CI can resolve the crate from crates.io.

Validation

Validated against the local frame-decode PR checkout using Cargo's command-line patch configuration:

• cargo check -p subxt
• cargo check -p subxt --no-default-features --features web
• downstream TrUAPI server suite: 434 tests passed
• downstream TrUAPI CLI createTransaction example: both V4 and V5 construction passed; V5 produced a General extrinsic with inner prefix 0x45

Downstream integration

paritytech/truapi#333 consumes this Subxt branch and the frame-decode change through immutable Git revisions. Its complete server suite, generated V4/V5 example checks, and focused CLI V5 transaction check pass.

Current Subxt CI failures are expected dependency-resolution failures: crates.io reports frame-decode 0.18.0 as yanked. The frame-decode release version must be moved to an available version, then this dependency requirement and lockfile must be updated in follow-up commits.