fix(metadata-protocol): render a composite externalId in seed diagnostics instead of its NUL-joined key

.changeset/seed-diagnostic-nul-joiner.md

@@ -0,0 +1,23 @@
+---
+'@objectstack/metadata-protocol': patch
+---
+
+Seed loader: a composite `externalId` no longer puts a raw NUL byte in a
+diagnostic line.
+
+`SeedLoaderService` joins a composite natural key's parts with U+0000 on
+purpose — that byte cannot occur in a natural-key value, so `('a','b')` and
+`('a\0b','')` never collide. The map key is unchanged. What changes is that
+the key string is no longer interpolated into human-readable messages: the
+`Failed to write <object> record #N (<fields>=<value>)` parenthetical and pass
+2's `on record '<value>'` lines now render a composite value as a JSON array of
+its parts (`(employer+user=["emp-1","usr-2"])`).
+
+A single-field `externalId` renders byte-identically, so non-composite
+diagnostics do not move, and the structured `errors[].attemptedValue` still
+carries the real key.
+
+Why it mattered: one raw NUL makes `grep` classify the whole server log as
+binary, so every later `grep -n` / `grep -c` over it silently returns nothing
+until the reader adds `-a` — the reader's main instrument disabled by one byte,
+at the moment someone is diagnosing a failed boot.