Store large build artifacts more efficiently

Large build outputs often change only in a few places, but traditional remote caches store every new version as a completely separate object. Kura now supports Bazel's content-defined remote cache chunking. The client splits a large output at stable content boundaries, uploads only chunks the cache does not already have, and records a compact recipe that reconstructs the original bytes.

This can reduce storage growth and upload work for successive versions of large artifacts while preserving the same verified content digest. Kura reconstructs downloads as a bounded stream, verifies every new recipe before accepting it, and safely falls back to a cache miss when a node cannot read the recipe. The capability is enabled by default for Bazel clients that negotiate it, while operators can temporarily disable new recipe writes during a reader-first fleet upgrade.