parse: align SEIPD v2 chunk size rendering

[spiegel-im-spiegel]

Summary

• align tag18 (SEIPD v2) chunk-size rendering with tag20
• decode chunk-size octet as 2^(c+6) and show the encoded octet as dump
• add a focused v2 test vector in tag18_test.go to lock expected output
• add docs/rfc9580-coverage.md as a coverage inventory for current 4880bis/9580 state

Why

• avoid inconsistent output semantics between tag 18 and tag 20 for the same AEAD chunk-size field
• preserve a concrete baseline document before broader RFC 9580 implementation work

Validation

• task --force test passed
• linter passed with 0 issues

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