perf(ext/fetch): cache Headers iteration for all guard types
The `_iterableHeaders` getter was rebuilding the sorted entries array on
every access for non-immutable Headers (guard !== "immutable"), which is
the common case for user-created Headers objects. This caused ~60x
slower iteration compared to a Map.
Cache the computed entries for all guard types and invalidate on
mutation (append, set, delete). This reduces the gap from ~60x to ~3x.
Closes #20980
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
