docs(link): the Protocol 3.0 cutover (v3.md order 6)

INTEGRATION.md was written for the window that just closed -- it told integrators
the version had NOT been bumped yet and that a sidecar on `edge` reports 2 while
already carrying v3 kinds. That guidance is now wrong in the direction that
matters, so the version section states 3 (header, /health, ws.hello, the 409
example and the §8 worked example) and replaces the "until then" paragraph with
what a v2 integration actually has to do to upgrade: change the constant it
sends, and nothing else, because nothing that existed in v2 changed shape.

v3.md gains §4.1 for what the bump touches and, more importantly, WHY the
website's boot migration is gated on a marker row: schema.sql is re-run on every
boot and uo_link_config.protocol is admin-editable, so an ungated UPDATE would
silently un-pin an operator running an older sidecar. That is the one piece of
the cutover a reader could not infer from the code being one constant.

Progress tables: 5b done, 6 in review.

Co-Authored-By: Claude <noreply@anthropic.com>

link/PLAN.md

@@ -347,6 +347,12 @@ of 25 vendors x 40 listings and **0.3 ms** in steady state (the per-vendor diff)
 items / 43k mobiles. It is also the first stream to honour a per-player privacy toggle: ServUO's own
 `PlayerVendor.VendorSearch` flag, so a shop hidden in game is hidden on the site.
 
+That completes 3.0's feature work, so the last step is the version itself: `PROTOCOL_VERSION` **2 →
+3** and the coordinated `edge` → `main` merge across all four repos ([`v3.md`](v3.md) §4 and §4.1).
+The bump is deliberately the *only* thing that happens at that moment — v3 adds kinds and endpoints
+but changes nothing that already existed in v2 — so the operator-visible break is limited to
+re-pinning the version, which the website does for itself in a one-shot boot migration.
+
 ### Config keys (`Config/Bridge.cfg`)
 
 ```ini