Test-migration: v1 diagnostics module (328 LOC, 12 tests)

[briansrls]

Summary

Migrate src/v1/tests/src/diagnostics.rs to floor witness dag/test/claim/diagnostics_test.dag (exact stem diagnostics), then delete the v1 module.

Not 1:1 test-count parity — curated four high-signal floor witnesses (host-physics via diagnostics_witness + shell transport):

1. diagnostics_import_resolution_holds — MissingExport message + span grounding + UnresolvedImport variant
2. diagnostics_reexport_surface_holds — variant not re-exported through type-only import surface
3. diagnostics_type_and_arity_holds — UnresolvedType + ArityMismatch on bare List but not List<Int>
4. diagnostics_empty_list_context_holds — empty-list literal fails without collection context, passes with List<String> context

Dropped (tautological/redundant): clean-compile zero-diags sanity, multi-missing-export span bookkeeping, user-type arity false-negative, duplicate_module_detected (#[ignore] PERF).

Test plan

• diagnostics_witness — all 4 suites pass locally
• claim_batch --claim-run ... diagnostics_test.dag --functions diagnostics_import_resolution_holds,diagnostics_reexport_surface_holds,diagnostics_type_and_arity_holds,diagnostics_empty_list_context_holds — 4/4 PASS
• cargo clippy -p v1-compiler --bin diagnostics_witness -- -D warnings — clean
• No 03_resolve.dag / 04_infer.dag changes

[gunbai-bot]

Addressed in e8cb3aa.

Witness #3 negative controls — both asserted, not just named:

type_and_arity_discrimination in diagnostics_witness.rs now exercises three ArityMismatch axes in one suite:

1. Positive: bare List → has_arity_mismatch must be true (lines 271–279)
2. Std-container negative: List<Int> → !has_arity_mismatch with failure message (lines 281–287)
3. User-defined-type negative: Widget field reference → !has_arity_mismatch (restored from v1 unknown_type_name_no_arity_false_positive; lines 289–296)

The reviewer is correct that (2) and (3) are distinct regression vectors — parameterized std containers vs user-defined nominal types. (2) was already asserted; (3) was incorrectly dropped as "redundant" and is now merged back into witness #3 rather than a separate floor row.

Provenance string updated to document both negative controls explicitly.

— sent from proud-hawk-812