Resolve shelf.io.connection_info lazily (#542)

`shelf_io` resolves the `shelf.io.connection_info` context value lazily instead
of while constructing every `Request`.

`dart:io` does not cache `HttpRequest.connectionInfo`. Each call constructs a
fresh `_HttpConnectionInfo` by reading `remoteAddress` and `remotePort` off the
socket (`_HttpConnectionInfo.create` in `sdk/lib/_http/http_impl.dart`), so
building it eagerly spent two `getpeername` calls on every request — including
the majority that never read the value. #37, which added the field, asked for it
for request logging, which most handlers do not do per request.

## Effect

200 requests, a fresh connection each (`Connection: close`), counted with
`strace -f -e trace=getpeername,getsockname`. The handler either ignores the
context value or reads `remotePort` from it.

| build             | getpeername | per request | getsockname |
| :---------------- | ----------: | ----------: | ----------: |
| `master`, ignored |         402 |   **2.000** |     7 total |
| `master`, read    |         402 |   **2.000** |     7 total |
| this PR, ignored  |       **0** |   **0.000** |     7 total |
| this PR, read     |         400 |   **2.000** |     7 total |

A handler that ignores the value pays nothing; one that reads it pays exactly
what it paid before. On a small-response workload where `shelf_io` issues ~14.8
syscalls/request, this removes ~13% of them for handlers that do not use the
value.

`getsockname` is 7 calls total in every arm — fixed startup cost, not
per-request. `socket.port` is served from cache and never reaches the kernel.

## Implementation

The context value is a private `_LazyHttpConnectionInfo implements
HttpConnectionInfo` holding the `HttpRequest`. It resolves all three fields
together on first access and caches the result.

`HttpConnectionInfo` is an interface with exactly three getters
(`remoteAddress`, `remotePort`, `localPort`), and the SDK's implementation is
private, so no caller can be depending on a concrete type.

If the connection is already gone, `HttpRequest.connectionInfo` is `null` and
the `!` throws. That error now surfaces when a handler reads a field rather than
while the adapter builds the `Request`.

## Tests

Existing coverage passes unchanged, including the `isA<HttpConnectionInfo>()`
assertion. One test added, covering `remotePort` (not previously asserted),
stability across repeated reads, and survival through `Request.change()` — since
middleware copies `context`, that is where a lazy value would break.

`dart analyze` clean, `dart format` clean, 232 tests pass.