Add pure server tests using lori's TCPBackend generic

[SeanTAllen]

Propagates lori 0.19.0's TCPBackend type parameter through HTTPServer, HTTPServerActor, and _ConnectionState with a RuntimeBackend default (no user-facing API change). Adds a package-private _install_send_capture hook on HTTPServer so pure tests can observe wire bytes handed to lori's send.

Replaces most of the TCP-backed server test suite with pure equivalents that drive HTTPServer[_TestFakeBackend] through _on_received, respond from on_request_complete, and assert on the captured wire bytes or lifecycle callbacks — no listener, client, port, or timeout.

Kept TCP-backed: the smoke suite (server/hello world, server/keep-alive), ASIO timer tests (need real timers), all SSL tests (TLS sits between HTTPServer and the backend), and the pure property tests that were already pure.

204 tests pass; lint clean.

Parked for your input

• Pre-existing HTTPServer bug surfaced by the adversarial review. HTTPServer.request_received overflow path (stallion/http_server.pony:312-321) calls _close_connection() but not _parser.stop(). Consequence: the parser finishes the overflow-rejected request, which calls HTTPServer.request_complete, which fires on_request_complete on the actor for the overflow-rejected request (the docstring at line 337-340 claims the else branch is unreachable — it isn't reachable, but the request_complete itself is reachable for the overflow case). parse_error sidesteps this by calling stop(). I did not fix this in this PR — it is a production behavior change outside the scope of "test refactor + type parameter". Filing as a follow-up unless you want it here.
• _TestFakeBackend naming. The API-design reviewer flagged that _Test is narrower than the reusable role. Rename to _FakeBackend (matching lori's _FBSendOk… style)? Cosmetic.
• Public asymmetric type parameter. HTTPServer[TCP] is a public class parameter, but every mechanism a caller needs to use it (_TestFakeBackend, _install_send_capture, _TestSendCapture, _SendCaptureNotify) is package-private. An external user can HTTPServer[MyBackend] and get no way to observe wire bytes. Options: commit to a public capability, hide the parameter, or leave it and say why it's asymmetric. Docstrings currently say "primarily for stallion's own pure-test suite" — is that the right message?
• Coupling documentation. When lori adds a method to TCPBackend, _TestFakeBackend must gain the same method or every pure test stops compiling. I documented this in _TestFakeBackend's docstring; should it also go in AGENTS.md?
• Alternative design. The _send_capture hook exists because TCPConnection[TCP] field-inits _tcp: TCP = TCP, so tests can't inject a pre-built fake to observe later. A lori change to accept a caller-built backend would let stallion drop the production-side hook. Worth exploring in a separate discussion.
• Pre-existing coverage gaps. No pure test hits _flush_data's SendError branch (fake sendv always returns OK); no test exercises lori's own throttle detection end-to-end. Both existed before this branch.