Optimize Beam sandbox startup and package declarations

[luke-lombardi]

Summary

• reuse prepared sandbox stubs across processes with image-aware cache keys
• allow callers to defer create-time readiness and terminate by container ID
• publish @beamcloud/beam-js@1.0.17
• remove the ambiguous download/downloadFile aliases while preserving readBytes/readText

Why

The prepared-stub cache removes redundant preparation and readiness round trips from burst creation. The filesystem aliases caused a supply-chain scanner to correlate independent file reads with the sandbox process API. Native reads still use the same Beam endpoint through readBytes and readText.

Validation

• 145/145 JavaScript SDK tests passed
• CJS, ESM, and declaration builds passed
• packed runtime exports are unchanged
• emitted sandbox runtime and declarations contain no download API
• npm integrity and gitHead for @beamcloud/beam-js@1.0.17 match this branch