Resolves #245

Problem

On the Node adapter, once the native (undici) Request materializes — via req._request, clone(), formData()/blob()/bytes()/arrayBuffer(), or any lazily-inherited native property — get headers() switched from the NodeRequestHeaders wrapper to #request.headers. A reference taken earlier became detached: req.headers !== h, and mutations through h were silently lost. The approach tried in #243 (keep the wrapper canonical forever) fixed identity but froze _request.headers / clone() at a materialization-time snapshot, so it was reverted (1754b02).

Fix

Instead of swapping which object the getter returns, swap the wrapper's backing store. NodeRequestHeaders already fronts a lazily-built native Headers; at materialization it now _adopts the native request's Headers and becomes a pure facade over it. From then on there is a single mutable header store, so with no dual-write bookkeeping:

• header references taken at any point stay live, and req.headers identity is stable forever,
• mutations through req.headers are visible in _request.headers, clone(), and formData()'s content-type sniff,
• mutations made directly on _request.headers are visible through req.headers (which one-way write-through could never give).

The untouched-headers hot path is unchanged; requests whose headers were accessed pay one extra branch + delegated call per header op after materialization.

Semantics note

After materialization the single store is the native request's Headers, which carries the fetch spec's "request" guard: forbidden-header mutations (e.g. set("host", …)) are silently filtered where the pre-materialization wrapper allowed them. This moves toward spec behavior and only applies once a native view exists.

Tests

• revives the reverted headers reference stays live across _request materialization test from fix(node): Node adapter correctness fixes (statusText, empty body, HEAD streaming, send errors, sync bridge) #243,
• its dual (the case that sank fix(node): Node adapter correctness fixes (statusText, empty body, HEAD streaming, send errors, sync bridge) #243): mutations after materialization reach _request.headers and clone(), in both directions,
• formData() honors a content-type set through req.headers before materialization.

Full suite: 1041 passed, lint + typecheck clean.

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