feat(cxf): replace the export panic with a typed ExportUnsupported rejection

[jscott3201]

Rung 4 of 7 of the interchange-trio ladder (owner-ratified 2026-07-21) — the #141 export floor. #141 stays open; R7 closes it.

What

• oce_cxf::export no longer panics: the unimplemented!() is replaced by the prior-adjudicated typed floor — Err(CxfError::Validation(...)) carrying exactly one Diagnostic::error(DiagCode::ExportUnsupported, ...) with subject None (honest for a whole-operation deferral) and the ratified message CXF export is not yet implemented; every model is rejected until the exporter lands. — the staged rejection surface R5–R7 reuse.
• DiagCode::ExportUnsupported added (additive under #[non_exhaustive]) with its "export-unsupported" arm in the wildcard-free as_str match.
• export's rustdoc now documents the staged contract (the Result shape is the permanent API); the crate docs no longer claim export panics. Signature unchanged.

Tests

3 new export-floor tests (import-then-export, repeated-call stability on the full Diagnostic, hand-built-graph input-independence) + the oce-diag as_str uniqueness/kebab pin extended. Workspace 1219 → 1222. Mutation self-checks: substituted DiagCode → 2 tests fail; Ok(Vec::new()) → all 3 fail.

Gates

Implementer + independent lead re-run, both green incl. both rustdoc gates: fmt · clippy · nextest 1222/1222 · doctests · file-size · doc gates · determinism mirror 448/448.

Flagged for backlog, not this PR: the pre-existing oce-diag as_str pin test omits seven older variants (backfill candidate); oce-docs::point_list_html carries its own deferred unimplemented! (M4 scope).

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