Conformance test for the client/server boundary, and raise UnsupportedReturnType instead of silently stringifying

[spMohanty]

The client forwards every op in its registry (iter_proxyable() — 473 of them) to the server without ever checking that the result can actually come back. _pack_result on the server is a type switch with a str() fallback at the end, so a return type it doesn't recognise just slips through and then dies in msgpack.packb with the opaque "failed to serialize response". That's exactly what random.default_rng did to participants until 0.4.2 (#109) — it returns a Generator, which hit the fallback. And that fix only patched default_rng; nothing stops the next unhandled return type doing the same thing.

So this adds a guard for the whole class of bug, and makes the failure loud instead of silent.

Server side. _pack_result now raises UnsupportedReturnType for results that aren't msgpack-native, instead of stringifying them. It comes back as error_type: "UnsupportedReturnType" with the offending type in the message. _make_serializable still flattens the nested-numpy cases, so anything that packed fine before is unchanged.

The test (tests/test_registry_conformance.py):

• test_op_round_trips runs a real call for each exercised op through an in-process RequestHandler and fails if the response is UnsupportedReturnType/FlopscopeServerError. Arg errors (ValueError/TypeError) get skipped — that just means my example args were wrong for the op, not that the boundary is broken.
• test_every_proxyable_op_is_classified is the guard: every proxyable op has to be either exercised or parked in PENDING_EXAMPLES. Add an op to the registry and it lands in neither, so this goes red until someone gives it an example.

There's also a test_default_rng_round_trips as an explicit regression for #109.

A few notes:

• ~225 ops are exercised right now (category defaults for unary/binary/reduction, plus hand-written args for the irregular ones); the other 248 sit in PENDING_EXAMPLES. Shrinking that set is follow-up work — the point here is that nothing falls off the radar silently.
• The RemoteGenerator methods (rng.standard_normal() and friends) aren't module-level proxies, so they're outside iter_proxyable() and this test's scope.
• One behaviour change worth a look: an op whose result used to get silently str()-ed will now error instead. I ran the full suite and nothing actually hit that path, but flagging it in case.

Ran locally: the conformance file is 227 passed / 0 skipped, the full pytest --cov=flopscope run is green, and sync_client.py --check, the version-sync check, ruff, and pyright are all clean.