feat: Collapsed Data Center Fabric JVD — docs, snips, BYOAI

[KB-x]

Changes

• Documentation corpus — faithful markdown of the published JVD-DCFABRIC-COLLAPSED-01-02 PDFs (newest revision): solution-overview, design-guide, test-report-brief, datasheet + topology figure. README gains a Documentation section.
• Snip library (6) — Junos-only: direct leaf-to-leaf eBGP underlay + EVPN overlay (the collapsed-distinctive part), VLAN-aware MAC-VRF, ESI-LAG access, anycast IRB gateway, loopback. Plus _variables.md + snips README.
• BYOAI bundle — slug collapsed, Configuration + Design modes, grounded in the snip library and doc corpus.
• Portal — registered in snips.json (623 snips / 15 JVDs / 0 warnings) with a byte-identical BYOAI mirror.

Verified before merge

• jvd_verify.py pre-PR gate GATE PASSED (roundtrip · Seen-on resolution · secrets · doc code/version consistency · links + anchors · prompt fence · mirror byte-identical · DEFAULTS vs configs · change set).
• Roundtrip validator PASS — 6 snips, 41 leaves, 0 misses.
• bun run build PASS; DEFAULTS device inventory (loopbacks/ASNs, VNI, ESI, anycast MAC) verified against the rendered configs; test-report platform models manually checked (all real).

Notes

• Grounded in the 01-02 revision (only diff from 01-01 is the Junos version bump to 23.4R2-S3).
• Collapsed = two switches performing spine + leaf + border-leaf roles, peering directly with no separate spine tier. For scale-out, see the 3-stage DC JVD.