fix(decode): accept signed enum tag scalars in _extract_tag

[Stevengre]

Summary

Change _extract_tag pattern match from signed=False to signed=_ so both signed and unsigned tag metadata are accepted. Add enum-3-variants-0-fields-signed-tag decode test fixture.

Context

Python's _extract_tag matched only signed=False in the PrimitiveInt pattern for enum discriminant scalars. However, some enums (e.g. 3-variant enums like Ordering) use signed: true tag metadata in SMIR — this comes from the Scalar representation where the compiler may choose a signed integer type for the discriminant.

Stable MIR enum discriminants represent raw tag bits. The signed flag describes the type metadata, not the encoding — unsigned decoding of the raw bytes is always correct regardless of the signed flag. Miri also reads discriminants as raw bytes without conditioning on signedness.

Without this fix, decoding a 3-variant enum raises:

ValueError: Unsupported tag: Initialized(value=PrimitiveInt(length=I8, signed=True), ...)

With this fix, the tag is correctly extracted as unsigned bytes regardless of the signed metadata flag.

Proof evidence

Without fix (RED): test_decode_value[enum-3-variants-0-fields-signed-tag] raises ValueError

With fix (GREEN): Test passes, decodes to Aggregate(variantIdx(1), .List) as expected.

Test plan

• test_decode_value[enum-3-variants-0-fields-signed-tag]
• make test-integration regression