Add a new how-to page covering core's RegisterServeMuxOption and
RegisterHTTPMarshaler hooks. Page contents:

- Overview of the two functions and the init-only constraint.
- Ordering rules table (last-write-wins vs additive options) with the
  WithIncomingHeaderMatcher caveat.
- Worked recipes:
  - MessagePack marshaler via shamaton/msgpack with a protojson bridge
    for proto WKT correctness.
  - Tuning the default runtime.JSONPb (EmitUnpopulated, UseProtoNames,
    Indent) by registering for runtime.MIMEWildcard.
  - Per-route middleware via runtime.WithMiddlewares (request size limit
    example).
  - Custom error handler via runtime.WithErrorHandler returning a
    vendor-specific error envelope.
- "When to reach for these hooks" guidance pointing back at
  Interceptors for gRPC-side concerns.

Also:
- howto/index.md — new index row linking to the page.
- config-reference.md — DISABLE_ZSTD_COMPRESSION and PREFER_ZSTD rows
  documenting the new core flags.
- tests/navigation.spec.ts — add /howto/gateway-extensions/ to the
  Playwright howtoPages list so CI asserts the page loads.

Addresses Copilot review feedback on PR #85. Match the established
quickstart pattern (cb := core.New(cfg) followed by cb.Run()) instead
of pointing readers at a non-existent core.Run() free function.

- Replace 'a package init' with 'a package-level init() function' since
  package init isn't a Go construct.
- Add a warning callout that the msgpack NewDecoder reads the full
  request body via io.ReadAll; pair with http.MaxBytesReader middleware
  (which the per-route middleware recipe already demonstrates).
- Drop the 'coldbrew' import alias and use 'core' consistently across
  all four PreStart snippets, matching the convention in quickstart.md
  and other how-to pages.
- Remove the unused [ColdBrew cookiecutter] reference link.

- Add missing 'context' import to the msgpack, JSONPb, and middleware
  PreStart snippets so they compile as pasted.
- Restructure envelopeErrorHandler to marshal the payload first and
  fall back via http.Error on marshal failure, before writing
  Content-Type or the status code. The previous version discarded
  the marshal error and could leave the client with status set but
  an empty body.

- Rename 'Recipe: Per-route middleware' to 'Recipe: Gateway middleware'.
  runtime.WithMiddlewares is mux-wide, not route-scoped; the old heading
  contradicted the body. Update the cross-reference link from the
  msgpack callout to match.
- Rewrite the JSONPb intro to clarify that the wildcard registration
  only overrides the *fallback* marshaler. Add a note that callers who
  bind application/json explicitly (e.g. USE_JSON_BUILTIN_MARSHALLER=true)
  need to register the tuned JSONPb for that concrete MIME too.
- Add a warning callout under the error-handler recipe noting that the
  map[string]any envelope only works for JSON-shaped marshalers.
  Proto-only marshalers (application/proto, application/protobuf) need
  a proto.Message such as status.Convert(err).Proto() or a custom
  generated proto envelope.

- Move the gateway-extensions row to the bottom of the howto index
  table to match its nav_order (20), keeping the index in sync with
  the sidebar order.
- Add /howto/local-dev/ to the Playwright howtoPages list. It's been
  linked from howto/index.md but missing from the smoke suite.

- gateway-extensions.md: fix 'register per-route middleware' wording in
  the Overview to 'register gateway middleware', matching the recipe
  heading downstream.
- gateway-extensions.md: rewrite the error-envelope warning callout.
  Only runtime.JSONPb and runtime.JSONBuiltin accept arbitrary Go
  values; the MessagePack recipe earlier on the page type-asserts
  proto.Message and would also reject map[string]any, so listing it
  alongside the JSON marshalers was wrong.
- tests/links.spec.ts: add /howto/ and /howto/gateway-extensions/ to
  pagesToCrawl so a typo in the index entry or the new page's outgoing
  links is caught by the link-crawl suite.
- tests/content.spec.ts: add /howto/gateway-extensions/ to the meta-
  description SEO list and the howtoPages list for the TOC test.

- gateway-extensions.md: add a precision-loss warning to the
  MessagePack recipe. The protojson-encoding/json-msgpack bridge
  decodes JSON numbers into 'any' as float64; int64/uint64 values
  greater than 2^53-1 lose precision through the round-trip. Point
  callers at a protoreflect-direct encoder for those fields.
- gateway-extensions.md: tighten the JSONPb intro to acknowledge that
  runtime.MIMEWildcard serves both inbound (Content-Type) and
  outbound (Accept) sides, and update the follow-up note to mirror
  that on the override case.
- gateway-extensions.md: rephrase the 'When to reach for these hooks'
  bullets so we don't have three consecutive 'You need …' bullets,
  and broaden the interceptors mental-model line to cover both
  server- and client-side interceptors (retries are a client concern).
- tests/links.spec.ts: extend pagesWithAnchors with the
  #recipe-gateway-middleware fragment so the new in-page anchor
  link is verified.
- tests/content.spec.ts: add /howto/local-dev/ to both
  pagesWithDescriptions (SEO) and the TOC howtoPages list. The page
  was already linked from howto/index.md and the navigation smoke
  list but was missing from the SEO and TOC suites.