feat(installer): add next integration lane (#686)

Add --next installer flag (build-from-source at the next integration branch; MOSAIC_NEXT=1 env equiv; explicit --ref wins). Three-lane install docs (stable @latest / --next prerelease / --dev source) + @next dist-tag pipeline design doc. Green PR-event CI 1626 + review-of-record APPROVE (head 3a5c12a5).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/design/prerelease-next-dist-tag-pipeline.md

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+# Planned Design — npm `@next` prerelease lane
+
+Status: **PLANNED / not yet built**
+
+## Current state
+
+`tools/install.sh --next` provides a prerelease integration lane by building the Mosaic CLI and gateway from source at the permanent `next` branch. This is correct for validating integration-branch source, but it is slower than the stable npm lane because it downloads the archive, installs workspace dependencies, builds packages, packs local tarballs, and installs those tarballs globally.
+
+## Medium-term target
+
+Publish every accepted `next` integration build to the npm registry under the `@next` dist-tag, for example:
+
+```text
+@mosaicstack/mosaic@0.0.49-next.1
+@mosaicstack/mosaic@0.0.49-next.2
+```
+
+Then move `tools/install.sh --next` from source-build behavior to a fast npm install:
+
+```bash
+npm install -g @mosaicstack/mosaic@next
+```
+
+The framework archive should still resolve from the matching `next` source/ref until framework packaging has a registry-backed equivalent.
+
+## Pipeline shape
+
+1. Trigger on successful CI for `next`.
+2. Compute the next prerelease version from the upcoming stable version plus a monotonic prerelease counter (`0.0.49-next.N`).
+3. Build and pack publishable packages in CI.
+4. Publish to the Mosaic Gitea npm registry with dist-tag `next`.
+5. Keep `latest` untouched; only main/release promotion can update `latest`.
+6. Teach the installer to prefer `@next` for the CLI/gateway prerelease lane once the registry tag is reliable.
+
+## Guardrails
+
+- `@next` is mutable prerelease convenience, not a deployment pin.
+- Stable installs continue to use `@latest`.
+- Contributor validation remains available through `--dev --ref <branch>`.
+- Pipeline must be reproducible and trace every prerelease package back to the source commit on `next`.