docs/nim: add eden test plan

Add pkg/pillar/docs/nim-eden-test-plan.md which catalogs existing unit and
eden coverage (including PR lf-edge#5901), enumerates uncovered code paths in
cmd/nim and supporting packages, and proposes a phased set of eden tests
that exercise startup ingestion, DPC verification and fallback against
real SDN-induced connectivity loss, persistence across reboot, LPS
overrides, the resolver cache, PNAC/bond metrics, WLAN with cipher
decryption, cluster static IP, KubeUserServices ACLs, LOC probing and
flowlog reconciliation.

Note in the document that Phase 1 of this plan is being implemented in
lf-edge/eden#1165 (open, draft); script names there may differ from the
names suggested here, so per-test entries should be revisited once that
PR merges.

Signed-off-by: eriknordmark <erik@zededa.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: eriknordmark