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/scratchpads/installer-next-lane-20260624.md

@@ -0,0 +1,35 @@
+# Scratchpad — installer `--next` lane
+
+## Objective
+
+Add a prerelease installer lane for the permanent `next` integration branch.
+
+## Scope
+
+- `tools/install.sh`
+- README/install documentation
+- Follow-up design note for future npm `@next` prerelease publishing
+
+## Plan
+
+1. Add `--next` and `MOSAIC_NEXT=1` as source-build shorthand for `next`.
+2. Preserve explicit ref precedence: `MOSAIC_REF` and `--ref` win over `--next`.
+3. Update installer source display/help text.
+4. Document three lanes:
+   - stable npm `@latest`
+   - prerelease `--next`
+   - contributor `--dev --ref X`
+5. Run shell and repo gates locally, then hold before push/PR until runner serialization greenlight.
+
+## Verification
+
+- `bash -n tools/install.sh` — pass.
+- `docker run --rm -v "$PWD:/mnt" -w /mnt koalaman/shellcheck:stable tools/install.sh` — pass.
+- `bash tools/install.sh --check --framework --next` — source display shows `ref: next, --next prerelease lane`.
+- `bash tools/install.sh --check --cli --next --ref feature-x` — source display shows explicit ref wins.
+- `MOSAIC_NEXT=1 MOSAIC_REF=feature-env bash tools/install.sh --check --cli` — source display shows explicit env ref wins.
+- `pnpm install --frozen-lockfile --prefer-offline --store-dir /home/jarvis/.local/share/pnpm/store` — pass (local override for repo `.npmrc` CI store path).
+- `pnpm typecheck` — pass (41 successful tasks).
+- `pnpm lint` — pass (23 successful tasks).
+- `pnpm format:check` — pass.
+- `bash tools/e2e-install-test.sh` — attempted; current baseline fails during gateway health after stable registry install because Valkey is unavailable in the clean container. The `tools/install.sh --yes --no-auto-launch` stage itself completed before the downstream gateway verification failure.