The per-home snapshot (`_SECRET_SOURCE_VALUES_BY_HOME`) deliberately also holds names the source
supplied but .env/shell WON (`skipped_existing`, #102041) — `build_profile_secret_scope` needs them.
Re-asserting the whole snapshot on a dotenv reload therefore froze a gap-filled or preserve-listed
name at its first-load value and a later .env edit lost, where a cold start would have honoured it.
`AppliedVar.authoritative` (= `override_existing` and not `preserve_existing`, computed where the
precedence is decided) feeds a per-home restore subset `_SECRET_SOURCE_RESTORE_BY_HOME`; the reload
re-asserts only that. The contributor's pre-load snapshot is dropped: nothing between the dotenv loads
writes the module dict, so reading the subset at the restore site is equivalent and skips the copy.
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