feat(testing): docker-based local nuget server for tier 3 channel

tests/integration/README.md

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+# Local NuGet server (Tier 3 release channel)
+
+A Docker-based NuGet server used to verify that `Fallout.*` changes work in a
+real consumer project **before** they merge — without polluting the
+GitHub Packages feed (Tier 2) with abandoned PR builds.
+
+Tracking issue: [#279](https://github.com/ChrisonSimtian/Fallout/issues/279).
+Channel taxonomy context: [#267](https://github.com/ChrisonSimtian/Fallout/issues/267).
+
+| Tier | Channel              | Audience            | Status                          |
+| ---- | -------------------- | ------------------- | ------------------------------- |
+| 1    | nuget.org            | production / users  | wired (#272, #273)              |
+| 2    | GitHub Packages      | beta / bleeding     | env exists                      |
+| 3    | **this local feed**  | pre-merge testing   | **minimum-viable (this PR)**    |
+
+## What this gives you
+
+A locally-running NuGet v3 feed at `http://localhost:5555/v3/index.json` that
+accepts `dotnet nuget push` (with an API key) and serves restores. Useful for
+manually validating a `Fallout.*` change against a real consumer shape before
+opening or merging a PR.
+
+The server is [BaGet](https://github.com/loic-sharma/BaGet) — an open-source
+.NET NuGet server with an official Docker image. Note: the BaGet repo is
+stale (last commit Jan 2023) but the image and NuGet v3 protocol surface
+remain functional. A follow-up may swap to a maintained alternative; the
+client-side workflow below is feed-agnostic.
+
+## Prerequisites
+
+- Docker / Docker Desktop installed and running.
+- .NET SDK matching `global.json` (for `dotnet pack` / `dotnet nuget push`).
+
+## Start the server
+
+```powershell
+# From the repo root:
+docker compose -f tests/integration/docker-compose.yml up -d
+
+# Verify it's up:
+curl http://localhost:5555/v3/index.json
+```
+
+The feed URL is `http://localhost:5555/v3/index.json`. Package data persists
+in the named Docker volume `fallout-local-nuget-data` across restarts.
+
+The API key defaults to `FALLOUT_LOCAL_DEV` (static, dev-only). Override via
+the `FALLOUT_LOCAL_NUGET_API_KEY` environment variable before `docker compose
+up` if you need a different one.
+
+## Push a package
+
+```powershell
+# Pack a Fallout.* project (example: Fallout.Common):
+dotnet pack src/Fallout.Common/Fallout.Common.csproj -c Release -o artifacts/
+
+# Push to the local feed:
+dotnet nuget push artifacts/Fallout.Common.*.nupkg `
+  --source http://localhost:5555/v3/index.json `
+  --api-key FALLOUT_LOCAL_DEV
+```
+
+To push every `.nupkg` produced by a full `./build.ps1 Pack`:
+
+```powershell
+Get-ChildItem output/*.nupkg | ForEach-Object {
+  dotnet nuget push $_.FullName `
+    --source http://localhost:5555/v3/index.json `
+    --api-key FALLOUT_LOCAL_DEV `
+    --skip-duplicate
+}
+```
+
+## Consume from a project
+
+Pick one:
+
+**Ad-hoc on the command line:**
+
+```powershell
+dotnet restore --source http://localhost:5555/v3/index.json `
+               --source https://api.nuget.org/v3/index.json
+```
+
+**Via `nuget.config` in the consumer project:**
+
+```xml
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <add key="fallout-local" value="http://localhost:5555/v3/index.json" />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+</configuration>
+```
+
+For PR-scoped builds, use a pre-release version suffix (e.g.
+`12.0.0-pr.NNN.YYYYMMDD.SHA`) so different PR runs don't collide on the
+same slot. Versioning automation is a follow-up — see "Deferred", below.
+
+## Stop the server
+
+```powershell
+docker compose -f tests/integration/docker-compose.yml down
+
+# Also remove the package data volume:
+docker compose -f tests/integration/docker-compose.yml down -v
+```
+
+## Deferred to follow-ups (tracked in [#279](https://github.com/ChrisonSimtian/Fallout/issues/279))
+
+- Wiring `tests/Consumers/Fallout.Consumer.Local/` to restore from this feed
+  automatically.
+- A `dotnet fallout` target (or `push-local.ps1` script) that packs all
+  `Fallout.*` projects and pushes them to the local feed in one command.
+- PR-scoped versioning convention (`Fallout.X.M.P-pr.NNN.YYYYMMDD.SHA`).
+- CI integration: GitHub Actions job that stands this up per-PR in an
+  ephemeral container and runs the consumer sentinel against it.
+- Devcontainer / Codespaces integration so this server is preconfigured
+  alongside the .NET SDK.

tests/integration/docker-compose.yml

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+# Tier 3 release channel: local NuGet server for pre-merge testing.
+# See ./README.md for usage. Tracking issue: ChrisonSimtian/Fallout#279.
+#
+# This compose file spins up BaGet (https://github.com/loic-sharma/BaGet),
+# an open-source .NET-based NuGet server. The repo is stale (last commit
+# Jan 2023) but the Docker image and protocol surface remain functional
+# for the dev/CI use case here. A future PR may swap to a maintained
+# alternative; the interface (NuGet v3 protocol over HTTP) is stable
+# regardless.
+
+services:
+  baget:
+    image: loicsharma/baget:latest
+    container_name: fallout-local-nuget
+    restart: unless-stopped
+    ports:
+      # Bind to localhost only — this is a dev/CI feed, never exposed externally.
+      - "127.0.0.1:5555:80"
+    environment:
+      # Static dev-only API key. Documented in README; do NOT reuse anywhere real.
+      ApiKey: "${FALLOUT_LOCAL_NUGET_API_KEY:-FALLOUT_LOCAL_DEV}"
+      Storage__Type: "FileSystem"
+      Storage__Path: "/var/baget/packages"
+      Database__Type: "Sqlite"
+      Database__ConnectionString: "Data Source=/var/baget/baget.db"
+      Search__Type: "Database"
+      # Reject unauthenticated pushes — clients must supply --api-key.
+      PackageDeletionBehavior: "Unlist"
+    volumes:
+      - baget-data:/var/baget
+
+volumes:
+  baget-data:
+    name: fallout-local-nuget-data