Show how a config's values compare to the compiled defaults

[andyyhope]

Alongside coverage, the mapping audit now diffs every value a config supplies against
the flag's compiled default — so a large file tells you what it actually does, not
just that it parses.

let audit = try FlagMappingAudit(AppFlags.self, applying: json)
print(audit.defaultsDescription)
// Default vs config:
//   changes (2):
//     • new-onboarding: false → true
//     • tier: "free" → "pro"
//   restated (2) — same as the default, could be omitted:
//     • markets
//     • page-size

• changesDefault — the values this config actually changes.
• matchesDefault — the values it restates identically to the compiled default, and
  could drop from a bloated file.
• defaults — the full per-flag comparison (FlagDefaultComparison), for asserting on
  a specific flag.

Informational only

Whether a value matches its default or differs from it, isComplete is untouched.
Restating a default is not a coverage failure, and neither is changing one — it is a
diff, for you to read and assert on as you like. Independent of absent: a flag the
config never supplies has no incoming value and does not appear in the diff.

Reuses the mapped values

It compares the boxes a real apply would store, not the raw JSON, so a whole number
sent for a Double flag matches a Double default — JSON's single number type does
not create a false change. A record list restated exactly matches its default, and
config field order does not matter, since both canonicalise to sorted-key JSON. Both
are pinned by tests — the record path is where a false change was most likely.

Verified

752 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