A missing Python is not a skip, it is UNSUP, and it blocks - #3

WHAT CHANGED
  test/canonical-float-policy.test.mjs   5 x `if (!python) return` -> requireCapability

WHY
  The file I added in 90a22a9 returned early when no python was found, which
  prints `ok` having asserted nothing. harness.mjs already has the mechanism for
  this, and its docstring says exactly what I did wrong:

    "It is NOT a skip. A test that returns early on a missing prerequisite
     prints `ok` having asserted nothing, which is the same false green
     ADR-0021 exists to prevent, one layer up."

  Worst in this file of all places: the defect under test IS a Node/Python
  divergence, so a green run with no python validates nothing at all while
  claiming the two languages agree.

  Mine was the only silent skip in the repository. The three older Python tests
  assert `python.status === 0`, so a python-less host fails them - blunt, but
  never a false green.

WHAT I VERIFIED
  node research-kit/bin/selftest.mjs   -> 565 passed, 0 failed
  node research-kit/bin/preflight.mjs  -> PASS, 0 blocking, 0 warnings

  Reproduced the missing-python condition by pointing the probe at an
  interpreter that does not exist:

    5 UNSUP  PYTHON-NOT-FOUND, blocking=5, exit 1 (selftest.mjs:56)
    1 ok     the structural scan, which needs no python and correctly still runs

  Attempted first by stripping PATH; on Windows the child still resolved python,
  so that reproduction was worthless and I replaced it with one that works.

  CI cannot hit this path: .github/workflows/offline-suite.yml pins
  actions/setup-python@v5 at 3.12 and runs `python --version` before the suite.

WHAT I DID NOT CHANGE
  The three older conformance tests. They block correctly already; converting
  them is a reporting improvement, not a defect fix, and belongs in its own
  change rather than widening this one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>