docs: clarify test-naming convention allows judgment for cross-cutting tests

The "one file per lexicon" rule is a default, not a strict requirement.
When a test exercises behavior that spans multiple lexicons or code
files (shared *.defs lexicons, cross-cutting union semantics, etc.),
pick the clearest name for what's under test rather than duplicating
near-identical tests across every carrier.

Cite validate-signature-defs.test.ts as an illustrative example.

Surfaced by CodeRabbit feedback on PR #170.

Author: aspiers

AGENTS.md

@@ -294,10 +294,25 @@ npm run check
 
 ### Writing Tests
 
-Tests live in `tests/` with one file per lexicon, named
+Tests live in `tests/` and by default use one file per lexicon, named
 `validate-<lexicon-slug>.test.ts` (e.g. `validate-link-evm.test.ts`,
 `validate-rights.test.ts`).
 
+This is a convention, not a strict rule. When a test genuinely
+exercises behavior that spans multiple lexicons or code files —
+shared `*.defs` lexicons referenced from many records, cross-cutting
+union semantics, validation behavior that surfaces only in
+composition, etc. — pick the clearest, most concise name that
+describes what's actually under test, even if that file doesn't map
+1:1 to a single lexicon. Don't duplicate near-identical tests across
+every carrier just to satisfy the naming convention.
+
+For example, `validate-signature-defs.test.ts` exercises
+`app.certified.signature.defs#list` (union of inline + strongRef,
+array semantics) via a representative record carrier, rather than
+spawning 19 near-identical files for every record lexicon that
+references the def.
+
 The generated code provides two ways to validate records:
 
 - **`validate()` from `generated/lexicons.js`** — generic, untyped.