[ADC-690] Prove multiphysics diagnostics and fail-closed layouts

[wolf75222]

Summary

• merge the current origin/master explicitly, preserving the three existing ADC-690 commits and their history
• keep six owner-qualified density/momentum diagnostics, including signed electron/ion charge contributions
• execute a real two-layout multiphysics preflight through the public LayoutPlanBuilder API and fail closed when the required ion-to-field mapping has no provider
• prove that this refusal creates no output artifact before running the valid native lifecycle
• include bind identity, layout-plan identity, and every materialized layout identity in the strict restart snapshot and bit-identical continuation comparison

User impact

The canonical multiphysics example now demonstrates both sides of the public contract: an invalid cross-layout coupling is rejected before state/effects can publish, while the valid same-layout transport/Poisson/collision case retains its existing HDF5, ParaView, checkpoint, and restart route. There is no provider substitution or compatibility fallback.

Current-head validation

• 30 focused Python/source tests passed across the ADC-690 example and final release-contract gates; the native direct-run test was deliberately excluded while another lane owns the native build
• Ruff passed on both changed Python files
• docs/check_docs.py passed with one existing freshness warning
• git diff --check passed

Explicit remaining work / non-claims

• clean native build/install and direct execution of EXEMPLE_SPEC_FINALE_MULTIPHYSIQUE_CORE.py have not yet been rerun on this merged head
• the missing mapping/provider negative is now executable and source-proven; a forced native non-convergence rollback with no publication is still outstanding
• PoPS still has no typed unit protocol, so runtime diagnostic units remain the exact fail-closed unspecified marker
• the signed owner-qualified charge terms are explicit, but no single cross-block derived scalar reduction is claimed

ADC-690 therefore remains In Progress and this PR remains draft.