[Kinflou]

#20 left six sites still answering a failure with ordinal: 0 and an
{"error": "…"} body, because each carried a message the user needs and a
fault is opaque on the wire, so there was nowhere else to put the text. This
adds that place.

Where the reason goes

SimOutcome::Fault now carries an optional note, and it never reaches the
wire. reply_now pins it to the response frame it just emitted, and
describe_frame hands it to the inspector as FrameDetail::note. The
simulator is both ends of the connection, so it can show what a real provider
would only have logged.

The note is a separate field, Frame::note, rather than reusing
Frame::fault. That field already means something else: what the wire's fault
injection did to a frame (dropped, corrupted, +40 ms).

Sites converted

• Forward with no connection picked
• begin_forward's refusals: no such connection, a refused connection, a
  forwarding cycle, a target without that function
• a relayed upstream fault, timeout or undecodable reply, now naming the
  connection that failed. A cycle reads forwarding cycle at the innermost hop
  and forward: <conn> faulted at each hop outward, which the test pins.
• a script that throws, errors at runtime, or does not compile
• the disabled-script stub

Before this, every one of these reached the caller as a schema error. Its
client decodes ordinal 0 as whichever declared error happens to hold it, so the
caller got a wrong answer rather than a failure.

A one-way bug these sites shared

A failing script on a one-way function emitted a response frame that no
caller was waiting for. fault_or_none now mirrors the existing ok_or_none:
a one-way call has no response frame to carry a fault, so it sends nothing.
That matches the runtime's rule that a one-way handler cannot fail.

Behaviour change

A script's throw used to raise ordinal: 0. It now faults, with the thrown
value as the note. It could never name a declared error anyway.

Verification

137 tests pass with the default script feature, 128 without. Clippy is clean
on both, and the crate builds for wasm32-unknown-unknown.