Epic: v4.1 usability layer over the pure fetch() core

[gmoon]

v4.1 — a usability layer on top of the pure fetch() core

v4.0 made the right correctness call: it replaced v3's response-mutating,
404-swallowing proxy.get(req, res) with a pure proxy.fetch(req) that
throws typed errors. But it threw out v3's ergonomics without a replacement,
and quietly narrowed header passthrough.

v4.1 keeps the honest core and rebuilds the missing usability as thin,
opt-in layers — each built on the one below, none polluting fetch():

fetch()      pure primitive — get a key, throw typed errors   (unchanged)
  └─ pipe() / middleware()   one-call ergonomics, honest status codes
  └─ staticSite()            index + error documents (S3 website parity)

Tracked work

• Restore header passthrough dropped in v4.0 (x-amz-meta-*, SSE, version-id) #125 — Restore header passthrough regression (x-amz-meta-*, SSE, version-id)
• Add pipe() / middleware() convenience adapter over fetch() #126 — pipe() / middleware() convenience adapter
• Add staticSite() — S3 static website hosting as an opt-in layer #127 — staticSite() static website hosting layer

Design principles

• The purity guarantee lives in fetch(); every convenience consumes it and
  never leaks response-writing or S3 error classification back into the core.
• Config for website behavior rides on a dedicated factory (staticSite()),
  not the constructor (Option 2) — so index/error-document logic stays in the
  layer.
• All examples are exercised by the smoke gate against a real bucket.

Implementation for all three is ready on a branch; PRs to follow.