0hmX/am3352

docs/ddr-unintended-same-net-contacts.md

# D12 same-net shortcut: serial length audit invalid

The eleven-host `ddr-facing-progress-45` candidate has an unintended D12 contact. Host `source_trace_690` and RAM1 saved escape `source_trace_629` explicitly share `source_port_1004` (RAM1 DQ4 / CPU DDR_D12). Host segment 10→11 on inner4 passes across the saved through via at (-10.000054, 4.500054) mm. Its centerline misses the via center by only 0.000054 mm; exact copper gap is -0.288546 mm, confirming overlap.

The nominal loop from the via through its saved exit and back along the host to the via projection is 9.450589834 mm: 4.173296985 mm of saved escape plus 5.277292849 mm of host. This is a closed redundant copper path, not a valid serial delay contribution. Other host/saved contacts nearby introduce additional shortcuts. Segment-summed package-to-package length therefore cannot certify timing; the loop number is a geometric witness, not an electromagnetic delay or unique shortest-path solution.

The all-host inventory contains 32 same-net contact pairs: 22 at host endpoints, and ten non-endpoint pairs, all on D12. The other ten hosts show only endpoint contacts in this check. Report: `dist/ddr-host-fixed-contact-audit/report.json`.

A strict routing acceptance gate should use the existing exact copper geometry and electrical ownership resolution:

1. Enumerate every host-to-fixed same-net contact, including full via annuli, trace widths, pads and overlaps. A zero centerline-intersection count is insufficient.
2. Allow only contacts at the two explicitly declared exit handoffs. Bind each exemption to the actual breakout identity, source-owned fixed trace, host endpoint, exact XY/layer and incident primitive. Merely sharing a net or being near an endpoint is insufficient.
3. Reject every additional fixed-copper contact. Validate endpoint continuation angles and reject extended overlaps beyond the immediate intended handoff.
4. Separately reject nonadjacent host self-contacts and unintended host-to-host same-net joins. A topology-aware graph must be acyclic between the source and sink before treating segment sums as the propagation path.

`reportSameNetContacts:true` exposes witnesses from `verifyDdrSystemCopper` without changing its default clearance behavior. `audit-ddr-host-fixed-contacts.ts` flags non-endpoint contacts; endpoint contacts remain explicitly labelled as requiring contract validation. This diagnostic is not yet the complete acceptance gate described above.