IR-550: cover admitted direct fairness refusal

[kreneskyp]

Summary

Completes the IR-550 direct fairness slice by exercising an IR-admitted temporal/fairness node as the requested call-node ID in the public exact-function generator. The test builds the package from the authoritative QSpec positive fixture at runtime; it does not copy a temporary QSpec or IR package into the repository.

The focused test verifies:

• The unmodified authoritative QSpec positive-all-families package admits through the public CheckedPackageV2::read path with the same evidence.
• The constructed fairness package is admitted through that same bounded public reader.
• Direct fairness request keeps its requested claim ID and returns typed Unsupported with the fairness node ID and temporal tag as its cause.
• The healthy sibling result, generated src/lib.rs, and location map are byte-identical when run alone or alongside the fairness request.
• A malformed fixture mutation that inserts null body.operation.laws is refused at /semantic_graph/nodes/0/body; the builder now avoids mutating absent keys.

Review and gates

• Independent combined code/Rust review: SR-8664, no findings.
• Independent diff-scoped gap analysis: SR-8665, no gaps in the slice.
• Pre-PR make -k ci: PASS on 07adadcc30b4b3505f26efdce5b638354f1d4a5b; 247 unit tests, 508 integration tests passed, 27 ignored, 1 doctest passed. Log: /tmp/ir550-prepr-ci-07adadcc.log.
• Pre-PR make kani-gate: PASS, 27/27 on the same head. Log: /tmp/ir550-prepr-kani-07adadcc.log.
• Pre-merge make -k ci: PASS on the unchanged clean head; MSRV and stable each passed 247 unit tests, 508 integration tests with 27 expected ignored, and 1 doctest; fmt, spec, lint, deny, unsafe audit, and rustdoc passed. Log: /tmp/ir550-premerge-ci-07adadcc.log.
• Pre-merge make kani-gate: PASS, 27/27 on the unchanged clean head with Kani 0.68.0 and CBMC 6.11.0. Log: /tmp/ir550-premerge-kani-07adadcc.log.

Together with merged PR #372, this closes the remaining direct fairness criterion in IR-550. IR-550 and the supporting IR-774 admission investigation can close after this merge is verified.

Refs IR-550