mod request_view

RequestView[origin] borrows method, URL, headers, and body from the connection's read_buf rather than owning them. For a 13-byte plaintext request the difference is invisible (a few String allocations vs. a few offsets); for a 1MB / 16MB multipart upload it's the difference between one memcpy of the whole body and zero.

Pieces in place after this commit:

• RequestView[origin] value type (this file).
• parse_request_view(data: Span[UInt8, origin]) raises -> RequestView[origin] parser that scans the request line + headers + body slice without per-header / per-token allocations.
• RequestView.into_owned() raises -> Request materialises a Request for handlers that need to keep request state past one event-loop iteration.

Pieces that come later (deferred to S3 follow-up — explicit notes in the relevant commit bodies):

• A ViewHandler trait whose serve_view takes RequestView[origin] directly. Today's Handler.serve signature stays as-is; the reactor's read path can adopt a view-based run_reactor_loop_view once the trait surface lands.
• Replacing _parse_http_request_bytes with the view parser inside the cancel-aware reactor path. The view parser exists here as a standalone public function; the reactor will adopt it once the trait integration is settled.

This intentional split keeps the diff reviewable. The shape and public API of RequestView are stable from this commit forward; the integration step is purely "switch the reactor's internal call site," which lands without breaking handlers.

Closes the type portion of Track 1.1; the integration portion moves to a follow-up alongside the reactor surgery.

Example:

var raw = "GET /a?q=1 HTTP/1.1
Host: x

".as_bytes()
    var view = parse_request_view(Span[UInt8, _](raw))
    print(view.method) # GET
    print(view.url) # /a?q=1
    print(view.headers.get("Host")) # x
    print(len(view.body)) # 0

    # Materialise an owned ``Request`` if you need one:
    var owned = view.into_owned()