prevent cross-architectural leakeage in matrix_job_re

[cgoea]

Motivation

status.json's matrix-cell key for PyTorch/JAX fan-out builds is parsed purely from job names as (py, ref) — it carries no GPU architecture. When a single build cell (e.g. py 3.12 | torch release/2.10) nests test jobs for multiple architectures (e.g. gfx942 and gfx1101), all of those jobs collapse into one shared, worst-of variant.

That conflated variant then gets broadcast onto every architecture's test leaf by _refresh_same_run_fanout_tests, so one architecture's failure silently drags down another architecture's otherwise-passing result (and vice versa) in status.json, even though the two GPUs' actual test outcomes are independent.

closes #75

Technical Details

• Added _TEST_ARCH_JOB_RE / _job_matches_arch to detect which architecture (if any) a job names in its own "Test | <arch>" segment, so a job can be classified as either arch-agnostic (the cell's shared build step) or scoped to one specific architecture.
• _variants_from_jobs, _derive_variants, and _create_leaf now accept an optional arch parameter that filters the job list down to that architecture's own jobs plus any arch-agnostic jobs before deriving matrix-cell variants.
• _refresh_same_run_fanout_tests now re-derives each existing test leaf's variants scoped to its own architecture, instead of reusing the build leaf's arch-blind variant list wholesale. It also rolls the leaf's status up from those scoped variants (previously it copied the raw run conclusion directly, which doesn't reflect the arch-specific outcome either).
• _merge_run_into_document now re-derives a per-architecture leaf when a single event legitimately reports more than one target architecture, closing the same conflation risk (and a related object-aliasing hazard) on the primary write path, not just the same-run refresh path.

Test Plan

• Added test_completed_fanout_build_does_not_leak_status_across_architectures, which sets up two architectures (gfx942 passing, gfx1101 failing) nested under the same (py, torch) build cell and asserts each architecture's test leaf reflects only its own outcome.
• Verified the new test fails against the pre-fix code (gfx942's leaf incorrectly flips to failure) and passes with the fix applied.
• Ran the full scripts/receive_therock test suite and checked for regressions against develop.

Test Result

• pytest scripts/receive_therock/tests/therock_update_status_json_test.py: 99 passed
• pytest scripts/receive_therock/tests/: 401 passed

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.