Diff record flags per record and field, not as a truncated blob

[andyyhope]

A record flag's default-vs-config diff was two .string(json) values, truncated to
forty characters and unreadable. It is now a structured, per-record, per-field
breakdown:

Default vs config:
  changes (2):
    • page-size: 10 → 25
    • payment-methods:
        ~ Apple Pay:
            enabled: true → false
            minimumSpend: 5.0 → 10.0
        - Bank transfer
        + PayPal

• ~ a changed record, each changed field on its own line, field: old → new. Only
  the fields that differ are shown; an unchanged record does not appear.
• - / + a record present on only one side.

Records are paired by their @FlagRecordKey when they have one — so a reordered list
is not a wall of false changes — and by position ([0], [1]) when they do not.

The structured form is public: each comparison's records is [FlagRecordDiff], with
.identifier and .change (.added / .removed / .changed([FlagFieldDiff])), so
you can inspect a specific record's specific field rather than read the text. Field
values render with a generous cap, not the forty-character one the whole blob hit.

Adversarial review

One defect, and it was the exact thing this feature removes. A config that sends the
same keyed records in a different order differs in its stored value, so it is a
change — but no record was added, removed, or edited, so the per-record breakdown is
empty, and the description fell straight back to the two truncated JSON strings. It now
says the list was reordered. Only reachable for a keyed list; an index-paired reorder
shows as field changes.

Also confirmed clean under review: a duplicate-key config is refused by the mapper and
never reaches the diff, and an emptied list reports every record removed.

Verified

760 tests on a clean build. DocC clean with --warnings-as-errors, both example apps
and the iOS package scheme build. Additive only — no breaking changes.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NnwCqKNm8rZ63Au6bpYdaD