fix(bb.js): cache compressed browser CRS

[Wondertan]

What

• Persist compressed BN254 G1 data in IndexedDB immediately after download.
• Reuse an exact cache entry or the prefix of a larger entry before fetching.

Why

Crs.new(size) can preload CRS without constructing the WASM prover, but the compressed browser path currently writes nothing to IndexedDB until srsInitSrs decompresses it. Navigation or reload after preload therefore repeats the large range request. This makes the existing CRS loader durable while leaving normal Barretenberg.new({ srsSize }) behavior unchanged.

Verification

• Added focused cache reuse and larger-prefix tests.
• cached_net_crs: 2/2 passing.
• Prettier and ESLint pass on both changed files.

CI is awaiting a maintainer-added ci-external or ci-external-once label.