Pieces are distributed as links, resolved lazily

Decision

Every piece is fetched as a single downloadable link to its .tgz, served by an engine-token, platform-scoped endpoint (GET /v1/engine/pieces/bundle?name=&version=) that 307-redirects to whatever source exists: a signed S3 object, the npm tarball for an official piece, or the app's file store for a custom (ARCHIVE) piece served directly. The box downloads the link and bun installs it, one path for all piece types, no piece bytes crossing the worker socket. S3 copies are warmed lazily: a miss returns the npm/file link immediately and fire-and-forgets a deduped SYSTEM job to cache the tarball.

Context

The old path branched registry-vs-archive and pushed custom-piece bytes over the worker Socket.IO connection (getPieceArchive, ProvisionInput.fetchArchive), coupling the execution box to the app connection. Making the box transport-uniform (ADR 0001's pure pool) required deleting that branch.

Why

"Everything is a link" makes the box transport-uniform over plain HTTP, deleting the socket byte path and the branch. Lazy beats eager:

• The warming job is deduped by jobId = bundle:<platformId|global>:<name>:<version>, so an on-demand per-piece job is safe and avoids a proactive "sync every bundle to S3" batch or a cold-start scan.
• Backing the link by npm or the file store makes S3 a pure optimization, so self-hosters with no S3 still get a working link.
• Rejected PR #13865's form: eager batch sync + global name@version S3 keys (cold-start cost, missing tenant scoping, weaker auth).

Consequences

Platform scoping is mandatory (data-isolation rule): the endpoint resolves via pieceMetadataService.get({ name, version, platformId }) and custom-piece S3 keys are platform-namespaced, closing the cross-tenant name@version collision from the original global-keyed PR (#13865).

• getPieceArchive / fetchArchive are removed.
• The link is the piece's own tarball, so bun install still needs npm egress for transitive deps.