PR: R4 format support — EncodingLayout and field-to-bit mapping #17

[metasmile]

Summary

Add proper EncodingLayout to VerificationSpec for RISC-V instruction encoding format support. R-type and R4 formats are now first-class concepts with explicit field-to-bit-position mappings.

Changes

• spec.rs: Added EncodingLayout struct with field_map: BTreeMap<String, FieldBitMapping> — format-agnostic field-to-bit mapping
• spec.rs: Added encoding: Option<EncodingLayout> to VerificationSpec
• xif.rs: YAML parser supports encoding: r (R-type, default) and encoding: r4 (R4 with func2)
• xif.rs: Built-in default layouts for R-type and R4
• cva6_xif_ref.xif.yaml: Added func2 field, encoding: r4, updated cross constraints

R4 format layout: {rs3[4:0], func2[1:0], rs2[4:0], rs1[4:0], funct3[2:0], rd[4:0], opcode[6:0]}

Verification

• cargo test --release: 14/14 pass, no regressions
• cva6_xif_ref: 262,144 total, 1,280 pass, 260,864 fail

Closes #17