[Compare] Diff engine: duplicate match keys silently vanish; nested-object values compare as equal

[Exotic209093]

Severity

Medium — two fidelity gaps in the shared diff engine (local file diff, snapshot compare, org compare).

What happens

1. Records load into a Map keyed by String(rec[matchField]) — Map.set keeps only the last row per key, no warning. The default org-compare key is Name, routinely non-unique: 3 rows named "Acme" → only one compared, summary.total undercounts, and with [Compare] Org-compare “Copy to target” inserts duplicates for Changed records #52 the wrong record can be synced.
2. Field comparison is String(sv ?? '') !== String(tv ?? '') — object values stringify to [object Object], so records whose nested values differ (Owner: {Name:'A'} vs {Name:'B'}) land in Unchanged.

Evidence

• src/ui/utils/dataDiff.ts:40-50 (duplicates), :73 (nested compare)
• tests/unit/dataDiff.test.ts:37 covers blank keys only.

Suggested fix

Detect duplicate keys and surface a count/warning in the result and UI; JSON.stringify non-null object values before comparing (and display them the same way).

Environment

Commit 74cf21b (main), WaveLink 0.6.0. Found in the 2026-08-31 adversarial audit (code-verified).

[Exotic209093]

Fixed in #121