Field mutation leaves derived caches and duplicated headers stale

## Problem

Several address, parameter and received fields retain derived parsed/rendered caches after direct value mutation. Header duplication can also share field state. Subsequent reads may return values derived from the old source or allow changes in one header to affect its copy.

Prepared commit `89f7b47dbc6c2db32c6ca687268a4309aac42feb` invalidates affected caches on mutation while preserving explicitly assigned display names, and gives duplicated headers independent fields.

External ownership/cache models cover repeated mutation, explicit names, duplicates and parameter fields. The unchanged 1,837-example suite passes on both Rubies; package and Action Mailer integration pass.

CONTRIBUTING requires regression specs, so this report does not open a source-only PR or modify tests.

Review prepared commit 89f7b47dbc6c2db32c6ca687268a4309aac42feb and its handling of mutation caches, explicit display names, duplicated headers, and parameter fields. Check the external ownership/cache models and regression coverage, then verify the unchanged 1,837-example suite plus package and Action Mailer integration; done means repeated mutation and duplication no longer retain stale or shared state.

Written by the indexing model from the issue text.