test: add diagnostic schema conformance snapshots

Summary

Add conformance snapshot tests for 10 core diagnostic codes. Each test is a minimal .0 source file paired with an .expected.json file that asserts the exact diagnostic output schema.

Diagnostic codes covered

app001, cgen004, err001, imp001, nam003, par100, std002, typ001, typ002, typ009

How it works

• Each conformance/diagnostics/<code>.0 is a minimal program that triggers that diagnostic
• Each conformance/diagnostics/<code>.expected.json asserts the diagnostic code, severity, and message pattern
• conformance/run.mjs is extended with a new diagnostics test type that validates snapshots

Validation

pnpm run conformance

All existing conformance tests continue to pass. New diagnostic snapshots are verified on each run.

Motivation

The diagnostic schema is a core agent-facing contract (see #173). Having snapshot tests for each code makes it easy to catch regressions when diagnostic output format changes across versions.