feat(cxf): export connector §7.4.1 attributes under the bare-scalar canonical subset (#141)

[jscott3201]

Rung 6 — connector §7.4.1 attributes: Bare-Scalar Canonical export

Interchange trio rung 6 of 7. Widens the R5 attr-free CXF export subset to emit connector §7.4.1 attributes under one canonical Bare-Scalar Canonical (BSC) wire shape, at the RT-2 render fixpoint.

What

• Emit (BSC, iff Some): S231:unit/quantity/displayUnit as bare strings (TermAttr::Bare) — Real only; S231:min/max as bare numbers (CxfValue::Float for Real, CxfValue::Int for Integer) — Real + Integer. Carried on port nodes. All-default connector emits zero attr keys → byte-identical to R5.
• Reject (ExportUnsupported, subject = owning block): nominal (R6-A), unbounded (R6-B), non-finite Real min/max (R6-C). The importer hardcodes nominal/unbounded to None; serde_json serializes non-finite f64 as null, which would silently break render(G1)==render(G2). The R6-C finite_real_bound guard is required for RT-2 (real_connector_bound stores Some(r) with no is_finite check → Some(NaN) is import-reachable), not defense-in-depth.
• Fold the two residual export rejection slices: subjectless OOR external_inputs, root-URN pre-seed collision.
• Zero importer changes — src/resolve/ and src/dto.rs untouched.

How

• classify_attrs replaces attrs_are_empty; safe total match attrs (no as_real().unwrap()); runs only in the attrs.matches(value_type) branch. build() stays infallible (emit_port_attrs, total match, no unwrap).
• MSG_ATTRS retired; the headline rejection test converted to the attr-bearing RT-2 acceptance test.
• Five removal-mutation pins, literal-text bit-pinned, each verified RED-then-restored-GREEN (lead verified R6-C first-hand; merge-review re-verified all 5).

Verification (lead, first-hand)

fmt · clippy · 1258/1258 nextest · doctests · both rustdoc doc gates · file-size · 448/448 determinism matrix · machete · no-secret · default-no-db — all green. Importer untouched. Golden connector_attrs.export.cxf.json is the BSC shape (bare min:0.0/max:350.0/unit:"K"/quantity:"ThermodynamicTemperature"/displayUnit:"degC").

Merge-review

3-lens Opus review: 3× MERGE, zero blocking findings (RT-2, subset/rejection, build-infallibility+determinism).

Closes no issue yet — R7 closes #141 (arrays flattened-form + enum deferral + enum-free G36).

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