Decouple the block proposer from the authenticated owner

[ma2bd]

Motivation

Stacked on #6589, which made the owner's signature over the whole block a first-class, self-verifying object carried in proposals and certificates. One restriction remains from before: a fresh proposal must be signed by the block's authenticated_owner themselves. This conflates two roles — authorizing the block's operations (and fee liability) and participating in a consensus round — and forces the client to re-sign pending blocks as the owner that staged them, track whether it still holds that key, and hard-error on pending fast blocks whose key was lost.

The block is the unit of authorization — a "transaction": the owner signs the entire proposed block, never operation subsets, so whoever carries that block to a round cannot alter what was authorized. Retries already work this way; this PR extends it to fresh proposals.

Proposal

• Drop the worker rule restricting explicit owner authorizations to retries: a proposal may now be signed by any owner eligible for the round, as long as the block's authenticated_owner (if any) has signed the block — exactly the check Retain the block owner's proposal signature alongside certificates (#456) #6589 already performs. The super-owner requirement for authorizations claiming the fast round is unchanged.
• Client: the staging owner signs the OwnerAuthorization at staging time — when their key is by construction available — and it persists in PendingProposal. Pending blocks are then always proposed under the current identity: the "re-sign as the staged owner" logic and the key-availability dance are gone from the main path (a fallback remains for pending proposals from older wallets), and the previously unrecoverable pending-fast-block error becomes reachable only for legacy data.

Test Plan

• test_pending_block_discarded_when_signer_key_missing is inverted into test_pending_block_proposed_by_other_owner_after_key_loss: the pending block staged by owner a now commits, proposed by owner b after a's key is withdrawn, and the resulting certificate retains a's authorization, which verifies against the block.
• cargo test -p linera-chain -p linera-core -p linera-rpc pass; clippy clean including --all-features.

Release Plan

• Backporting is not possible but we may want to deploy a new devnet and release a new SDK soon.

Links

• Stacked on Retain the block owner's proposal signature alongside certificates (#456) #6589 (owner authorization carried in proposals and retained in certificates)
• reviewer checklist