feat(core): Add CoreSMT verification pipeline with incremental solver and diagnosis

[MikaelMayer]

Problem

Strata Core's existing verifier generates all VCs upfront via symbolic execution, encoding each to a separate SMT file. This prevents incremental verification where solver state is maintained across statements.

Additionally, Core.ExpressionMetadata was Unit, discarding source location information and making it impossible to report accurate positions in verification failures.

Solution

Core Expression Metadata: Unit → SourceRange

Core.ExpressionMetadata is changed from Unit to Strata.SourceRange so that:

• Source locations are preserved through the B3→Core→CoreSMT pipeline
• Diagnosed failures can be converted back to B3 with accurate source positions

All code constructing Core expressions with () now uses SourceRange.none or a proper range. The SyntaxMono macro (eb[...]) gains a defaultMetadata field on MkLExprParams (defaults to Unit.unit for generic params, overridden to SourceRange.none for Core). TermType.toSMTString is extracted as a pure function to avoid duplication.

CoreSMT Verification Pipeline

An SMTSolverInterface with push/pop support enables incremental verification. A diagnosis engine splits failing conjunctions and checks each conjunct individually to identify which assertions cannot be proved or which covers are refuted.

B3 Verifier Migration

The B3 verifier now uses the CoreSMT pipeline: B3 expressions are converted to Core (preserving source locations), verified via CoreSMT, and diagnosed failures are converted back to B3 for display.

Changes

• Core.ExpressionMetadata: Unit → Strata.SourceRange
• SyntaxMono macro: defaultMetadata field for type-appropriate metadata
• TermType.toSMTString: extracted pure function to TermType.lean
• isCoreSMT predicate, SMTSolverInterface, diagnosis engine, VCResult.diagnosis
• B3 verifier migrated to CoreSMT pipeline
• C_Simp/Verify.lean: csimpMetaToCore conversion (Unit → SourceRange)

Testing

All existing tests pass with the new pipeline.

[MikaelMayer]

Solid PR. The CoreSMT pipeline is well-structured with clean separation of concerns across State, IsCoreSMT, ExprTranslator, Diagnosis, StmtVerifier, and Verifier. The B3→Core→CoreSMT roundtrip with source location preservation is a meaningful improvement. A few issues to address before merging.