[WS2] Tolerance contract alignment & WS2 drift report format #116

Part of #83 · depends on #108 · related to #102

A small but cross-cutting issue: make every WS2 test (TP, SP, cross-config, chain test) judge "aligned" against the SAME standard, and emit drift reports in a SAME format, so failures across WS2 are comparable and debuggable.

Why separate: #102, the TP/SP/cross-config issues each touch tolerance and drift reporting. Without one shared contract they will each define their own, and WS2 results become incomparable.

Planned PRs:

• Adopt the [WS1] Ground-truth harness + numerical contract for batch-invariant ops #108 per-dtype pass/fail threshold table as the single source of truth for all WS2 tests; document how each WS2 test references it.
• Define a shared drift-report format: max abs / relative error, token position, target id, owner rank, TP/SP/DP size, dtype, backend, reduction name (aligned with the format proposed in [FEAT] TP-invariant reductions for FSDP(TP=1) vs TP>1 rollout/training parity #102).
• Provide a shared assertion/report helper that all WS2 tests import, so no test rolls its own tolerance or report.