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feat(web): remove the X-Server-Function-Instance header from the wire protocol (#3421)

Since #3094 the header decided nothing: the answer shape is the address
(/data/ for the scripted transport, bare for plain HTTP), no-JS gating is
the address plus a form post, and #3416 had already dropped it from GET
reads so a <link rel="preload"> could match them. What remained was a
per-call id the handler copied into `context.instance` for the
transformResult / transformFlightResult hooks, which no consumer reads —
not the router (@solidjs/router@next), not frames, not the invocation or
trace channels. Cross-wire correlation is the trace context's job (#3402).

The header is gone from shared.ts, no longer sent by the client or read by
the server, and no longer exported from either entry; `context.instance`
leaves the hook contexts. The prepareRequest validation (#3174) uses the
transport's method as its sentinel on every call shape — the one field
every call carries — so a hook returning a fresh `{ headers }` is still
refused before dispatch. Client-side call observers keep their local
`instance` id.

Tests: 29 spec files stop emulating the header in hand-built requests. The
#3094 pins that sent the header to prove it was inert go with it — the
code path they pinned left in #3102, and there is nothing left to be inert
to; "shape by url alone" keeps its data-vs-bare comparison. The #3406
addressing test asserts the full header set per call shape.

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

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