docs(installer): add the operator install guide (Phase 0.4)

Closes the last Phase 0 item. INSTALL.md is written before the installer
binary on purpose: everything it installs is already released (0.1-0.3), so
the guide is not speculation about a tool that might exist - it is the
specification of what the run asks, where it writes, what it prints, and what
the operator does next.

It is useful today. Appendix A is the same deployment done by hand - bundle
fetch, tarball verify and overlay copy, the optional patch tier,
--print-config provisioning, systemd unit / sc create - composed from the
released artifacts' actual contents and the sidecar's config and CLI source.
That appendix doubles as Phase 1's acceptance test.

Writing it settled four things the plan had left implicit, now recorded in
PLAN.md:

- The installer does not install itself; day-two commands run from the
  downloaded binary.
- The flag surface: --servuo, --patches/--no-patches, --host, --site-url and
  --yes join the --verify/--bundle/--purge the plan already named, so every
  prompt has a non-interactive equivalent.
- A modified Config/Bridge.cfg is reported, not overwritten - one deliberate
  deviation from deploy.ps1, whose overwrite-on-hash-differs rule is right for
  a developer and would silently revert an operator's whole shard config on
  update. install.json's recorded hashes are what make the distinction
  possible.
- Remote-website deployments: widen [web] bind, firewall it to the site's
  address, front it with TLS or a VPN off a trusted network. [shard] bind
  stays on loopback because that socket carries commands into the game.

Also adds the missing installer/ section to the docs index, and refreshes two
stale examples in PLAN.md (overlay file count, sidecar version).

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

README.md

@@ -7,10 +7,11 @@ so they live in one place, independent of either codebase.
 ## Layout
 
 ```
-website/   docs from the shard website (Node/Express + MariaDB + React/Vite)
-link/      docs from the ServUO bridge (C# plugin + Rust sidecar + Node WS)
-android/   docs from the native Android client (Kotlin + Jetpack Compose)
-ci/        cross-cutting CI/quality notes
+website/    docs from the shard website (Node/Express + MariaDB + React/Vite)
+link/       docs from the ServUO bridge (C# plugin + Rust sidecar + Node WS)
+android/    docs from the native Android client (Kotlin + Jetpack Compose)
+installer/  docs for the installer that deploys a shard's bridge components
+ci/         cross-cutting CI/quality notes
 ```
 
 ### `website/`
@@ -50,6 +51,12 @@ ci/        cross-cutting CI/quality notes
 | [TRUSTED_DEVICES_APP_HANDOFF.md](android/TRUSTED_DEVICES_APP_HANDOFF.md) | Trusted-devices app handoff notes |
 | [PROJECT_TREE.md](android/PROJECT_TREE.md) | Auto-generated snapshot of the repo's tracked file layout |
 
+### `installer/`
+| Doc | What it covers |
+|---|---|
+| [INSTALL.md](installer/INSTALL.md) | **Operator guide** — installing Runic Gateway on a ServUO shard, connecting it to the website, and diagnosing it. Includes the by-hand path, which works today |
+| [PLAN.md](installer/PLAN.md) | Installer design of record — phases, locked decisions, the bundle/compat-matrix model |
+
 ## Provenance
 
 - `website/*` was extracted from `RunicGateway/website` via `git filter-repo`.