docs(link): add the Protocol 3.0 design

Surveys the live ServUO tree against everything the bridge already
surfaces and records the full gap list (17 items), then specs the four
features scoped for 3.0.

3.0 has three scope areas:

- A: the visibility framework. Admin-configurable, per-feature and
  per-field audience control over all ten shard-derived surfaces (the
  four new ones plus the six that already ship), on an
  anonymous -> logged_in -> player -> staff -> admin ladder. Every
  default reproduces today's behavior, so the retrofit is a no-op until
  an admin changes something. Two rules an admin cannot override: acct
  and webId are admin-only always, and an unmapped event kind is never
  broadcast below admin. This also fixes a verified leak - guild leader
  acct/webId are readable today on the anonymous /public/shard/guilds.
- B: three new wire streams - world.ruleset, points.board, and
  vendor.listing/vendor.listing.remove.
- C: the spawn atlas, built from static ServUO data files with no wire
  involvement.

Visibility lives entirely on the website; the sidecar stays a dumb
forwarder that defines no access parameters and advertises no
capabilities.

PROTOCOL_VERSION goes 2 -> 3 once, at the end: every part PRs into an
edge branch per repo, and the coordinated edge -> main merge is the
cutover. A schema migration moves uo_link_config.protocol so operators
don't have to.

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

README.md

@@ -27,6 +27,7 @@ ci/        cross-cutting CI/quality notes
 |---|---|
 | [INTEGRATION.md](link/INTEGRATION.md) | How the website integrates with the uo-link sidecar |
 | [PROTOCOL_2.md](link/PROTOCOL_2.md) | Protocol 2.0 / 2.1 design |
+| [v3.md](link/v3.md) | Protocol 3.0 design — shard content/standings streams + the visibility framework |
 | [ADMIN_CONTROLS.md](link/ADMIN_CONTROLS.md) | Staff write-plane (kick/ban/broadcast, page queue) |
 | [SHARD_PREREQS.md](link/SHARD_PREREQS.md) | Shard-side prerequisites for the bridge |
 | [PLAN.md](link/PLAN.md) | uo-link build plan |