feat(sw): parallel byte-range fan-out, streaming decrypt, Cache API persistence

[MichaelTaylor3d]

Summary

Closes the two performance items tracked in dig_ecosystem#4 (migrated TASKS.md #205d/#205e):

• #205d — parallel range fan-out + streaming decrypt. sw.js fetched a resource's byte-range
  windows one at a time and only responded once every chunk had been decrypted into one buffer.
  Window 0 is still fetched alone (it's the only way to learn the resource's total length), but
  every remaining window — on both the cacheable GET content path and the POST JSON-RPC fallback —
  is now fanned out concurrently (bounded to 6 in flight) instead of sequentially. Decrypt output is
  now a ReadableStream: chunk 0 is still decrypted eagerly (before any Response exists, so the
  wrong-key/decoy case still yields a clean 404 exactly as before), and chunks 1..N decrypt lazily as
  the stream is pulled, so a large multi-chunk resource starts flowing to the browser before the last
  chunk finishes decrypting.
• #205e — Cache API persistence. The verified dig-client WASM is persisted in the Cache API keyed
  by its pinned SHA-256 — a hit skips the fetch and lets the browser reuse the compiled-code cache it
  automatically associates with a Cache Storage entry (instantiateStreaming), instead of
  recompiling on every load. A decrypted PINNED (concrete-root) resource is persisted keyed by store
  id + root + resource key, so a repeat load — even a fresh SW instance / new browser session — skips
  the network round-trip and decrypt entirely. An unpinned "latest" read is never persisted this
  way, since that could serve stale content forever after the value changes — the whole point of the
  chain-anchored-root pin.

Security invariants preserved, not weakened: the merkle inclusion proof still requires (and gets)
the fully-reassembled ciphertext before anything is trusted — its leaf is a hash of the whole
resource, so it cannot be verified incrementally regardless; only the network round-trips leading up
to it are now concurrent. Every chunk is still decrypted through its own AEAD-authenticated
decryptChunk() call — nothing is skipped. The wasm SHA-256 fail-closed gate is unchanged (hash
verified at write-time; a same-origin, SW-private Cache Storage entry is trusted on read exactly like
an already-verified in-memory module would be). Full reasoning is in the doc comments in assets/sw.js
and in SPEC.md §9 (new).

What changed

• assets/sw.js — parallel window fan-out (planWindows/runParallel), streaming decrypt
  (buildDecryptStream), WASM + decrypted-content Cache API persistence.
• test/ (new, no dependencies) — Node-native (node --test) unit suite for the range planner,
  bounded parallel fan-out, cache-key builders, and decrypt-stream assembly, against a deterministic
  crypto stub (test/stub-dig-client.mjs); real AEAD/merkle correctness stays covered by digstore's
  dig-client-wasm Rust suite. test/load-sw.mjs documents how assets/sw.js (a browser-only ES
  module) is loaded under plain Node for these tests.
• .github/workflows/ci.yml — new js test job running the suite (Node 22, no install step).
• SPEC.md — new §9 documenting the parallel-fetch/streaming-decrypt/persistent-cache behavior as a
  normative contract; §8 clarified to scope "byte-identical" to the external route/wire contract, not
  sw.js's internal strategy.
• README.md / runbooks/local.md — mention the new test/ suite and local test command.
• Cargo.toml / Cargo.lock — 0.1.0 -> 0.2.0 (minor: new capability, no behavior-breaking
  change to any route, status code, or wire format).

Verification

• node --test test/sw.test.mjs — 20 passing tests (range planner, parallel fan-out concurrency
  bound + error propagation, cache-key builders, single/multi-chunk/failing decrypt-stream assembly,
  plus the pre-existing pure URN/header helpers now covered for the first time).
• cargo test --all-targets — 29 passing (unaffected; no Rust source changed).
• cargo clippy --all-targets -- -D warnings / cargo clippy --bin bootstrap --features aws — clean.
• cargo fmt --all --check — clean.

Test plan

• node --test test/sw.test.mjs green locally
• cargo test --all-targets green locally
• cargo clippy (lib + bootstrap --features aws) green locally
• cargo fmt --all --check clean
• CI green on this PR (rust lint + test, js test, cargo-deny, terraform fmt + validate, commitlint, version-increment)