fix(BACKEND-ROCM-QUANT-GATHER): label the comparison scope from the run, not a literal

Row: BACKEND-ROCM-QUANT-GATHER

tools/rocm_quant_gather/compare.py writes a fixed scope string into every report:

"scope": "synthetic operation parity; original fixture and model gates remain separate"

The original-fixture coverage run over the 32 verified Isotr0py/test-gguf-sample fixtures therefore produced a report whose own scope line calls it "synthetic operation parity". The run itself is correct (320 outputs, 140 byte-exact, all within the upstream atol=0.01 rtol=0.04); only the label is wrong, and the report is cited as row evidence.

Evidence: /home/vikash/.cache/rdna3-gather-oracle-impl/original-fixture-coverage/compare/report.json (report scope field) against its inputs (fixture-manifest.json/manifest.json, seal f83b5c3e5cdb988b623c1b7e565e325a9ed9ea8085ea132bc8e08e6978bc1bbb, exported from the 32 fixtures at revision d82b8773934ef260d8d8a896a7c197bc69a0fac1).

Fix in flow: make the scope a command-line argument whose default preserves the previous text, and re-run the comparison so the report's label matches the run's inputs. No comparison, tolerance, or gate semantics change.

Start in tools/rocm_quant_gather/compare.py and inspect how the report scope is written and how comparison arguments are parsed. Add the scope argument while preserving the current default, then re-run the comparison using the original-fixture inputs. Done means report.json labels the run according to its inputs without changing comparison, tolerance, or gate semantics.