## Why

The run build/download flow is split across Platform, platform_web, local debug scripts, and the independent run checkout. The editable run source may live at `browser/run` for convenience, but that directory must be treated as source input only. Local debug must snapshot or upload that source into an ignored, closed build bucket, let Platform dispatch `distribution.build`, build from the bucket, and prove the generated artifact can be downloaded and executed/updated without using the editable source tree as the runtime artifact.

## What Changes

- Treat `run/` under this repository root as the editable independent run source checkout for local debug, while keeping it ignored by the browser repository and still owned by `git@git.npc0.com:admin343/run.git`.
- Snapshot the editable run source into a closed ignored local build bucket before starting build-capable local debug flows; `RUN_BUILD_SOURCE_ROOT` must point at that bucket snapshot, not the editable checkout.
- Build the local bootstrap run worker from the bucket snapshot instead of executing `browser/run` in place; generated run packages remain Platform-dispatched `distribution.build` artifacts.
- Harden run distribution build workspaces so build output is isolated by plugin and job, not by server-wide mutable directories or editable source folders.
- Add tests that prove two servers for the same plugin can generate separate run distributions without artifact/config/key/result cross-talk.
- Upgrade local smoke proof so `scum-alpha` run generation is mandatory when the run endpoint advertises `distribution.build`, then download the generated artifact and verify safe metadata.
- Document every configuration value operators must provide or may tune for local debug and run distribution builds.

## Capabilities

### New Capabilities
- `run-build-download-flow`: Covers local-debug source snapshotting, closed build buckets, plugin/job-scoped run package builds, browser-safe run artifact downloads, and same-plugin multi-server build isolation.

### Modified Capabilities
- `run-distribution-and-client-managers`: Completed implementation must use the current-directory run checkout only as source input and prove generated run artifacts are downloadable.
- `artifact-transfer-channel`: Completed implementation must prove browser downloads and run artifact uploads remain chunked, checksummed, and free of leaked host paths or secrets.

## Impact

- Affected roots: `scripts/`, `platform/`, `platform_web/`, and the ignored independent checkout at `run/`.
- Affected local configuration: `RUN_SOURCE_DIR`/legacy `RUN_REPO_DIR`, `RUN_BUILD_BUCKET_ROOT`, `RUN_BUILD_SOURCE_ROOT`, `RUN_BOOTSTRAP_BIN`, `RUN_WORKSPACE_ROOT`, `RUN_SPOOL_ROOT`, `RUN_MAX_JOBS`, `RUN_PLATFORM_URL`, `RUN_ENDPOINT_ID`, platform storage/artifact paths, and bootstrap credentials.
- Verification requires structure checks, Platform tests, run tests from `run/`, frontend tests where touched, OpenSpec validation, and local debug smoke evidence.