Update ERC-8180: rename message-ID field author → sender

[0xrcinus]

Summary

Naming-consistency fix on ERC-8180. The Message struct in IERC_BAM_Decoder, the message-hash formula, and most surrounding prose already use sender for the message-sender's address. Only the message-ID formula, the MessageExposed event's third indexed topic, and a handful of prose references still used author. This PR renames those to sender.

• Message ID formula: keccak256(abi.encodePacked(author, …)) → keccak256(abi.encodePacked(sender, …))
• MessageExposed event: address indexed author → address indexed sender
• Test-case table column header and example formula
• Definitions list, prose references, Worked Example 4, Rationale, Security Considerations

Why this is zero-risk

abi.encodePacked doesn't see Solidity variable names — only the byte layout. The hashed bytes are identical before and after this rename, so:

• All previously computed message IDs remain valid
• All previously computed signatures remain valid
• The Test Cases table results are unchanged
• No deployed contract behavior changes

This is a documentation-only fix that eliminates a long-standing inconsistency between the Message struct field (sender) and the consumers of that field (formerly author).

What this PR preserves

• The frontmatter author: field (lists EIP authors — unrelated)
• The word "authorship" used as a concept ("per-message authorship verification") in §Behavior

Test plan

• Diff is 16 insertions + 16 deletions, all author → sender in the right places
• Frontmatter author: field preserved
• "authorship" (the concept) preserved
• Message hash formula unchanged (already uses sender)
• Signing domain unchanged
• No interface signatures change shape — only the third indexed topic of MessageExposed is renamed

Sequencing

This is PR 1 of a planned series of amendments to ERC-8180. PR 2 (contentTag promotion) will stack on top of this branch.

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