enhancement(state-manager+adversary): flag combined-footnote amendment sites as structurally coupled so partial edits can't silently drop cross-references

## Summary

When two co-located findings are remediated at a single amendment site, the resulting "combined footnote" fuses two independent cite chains into one parenthetical block. Nothing marks the site as structurally coupled, so a future edit that strikes or refactors ONE half of the footnote will silently drop the other finding's cross-references — and no current check would catch it.

**Severity:** LOW (enhancement / future-proofing — observed as a novel remediation shape, not yet as a defect)

## Evidence (ArcavenAE/switchboard-blue, vsdd-factory 1.0.0-rc.21, Phase 5 Pass 37)

Adversarial sidecar observation O-P5P37-A-001 (Adv-A lane, NO_FINDINGS pass):

- Burst 85 established the single-footnote-per-finding pattern (Ruling-14 §10).
- Burst 87 introduced the combined-footnote shape: the amendment at wave-6-tranche-a-scope-rulings.md Ruling-12 §1 fuses the F-P5P36-A-001 phantom-code redirect (with its dispatch-vs-transport precision paragraph) AND the F-P5P36-A-002 authorship-premise sibling amendment into a single parenthetical block.
- The sidecar's own assessment: "a future adversarial pass working on a Ruling-12 amendment will need to preserve the fused shape — striking or refactoring one half of the footnote without the other would silently drop cross-references."

The observation converged independently in the Adv-B lane the same pass (O-P5P37-B-001) — two isolated model families flagged the same structural-coupling risk, which is decent evidence it's real.

## Root cause

The remediation protocol treats amendment sites as per-finding; when two findings share a site, the fusion is ad-hoc prose with no machine-visible marker. Downstream editors (agents or humans) have no signal that the block carries two independent obligations.

## Proposed fix (either is sufficient)

1. **Marker convention (cheap):** state-manager's remediation protocol emits combined footnotes with an explicit inline marker, e.g. `` or a `[coupled-amendment: F-A, F-B]` prefix. Adversary prompts gain one line: "combined/coupled amendment sites must be edited as a unit; verify both cite chains survive any edit touching one."
2. **Protocol rule (structural):** disallow fusion — when two findings share an amendment site, require two adjacent footnotes (extends Burst-85 single-footnote-per-finding to a hard rule). Costs a little verbosity, removes the coupling class entirely.

Preference mildly toward (2): the single-footnote-per-finding invariant is already established and machine-spottable; the combined shape is the deviation.

## Environment
- vsdd-factory 1.0.0-rc.21, project ArcavenAE/switchboard-blue, Phase 5 Pass 37 (2026-07-04), observation convergent across both adversarial lanes