fix: methods returning Promise<RpcStub<T>> no longer produce stub-of-stub result types - #253

Apply ElideStub -- the same transformation Result applies to a declared
stub return -- to the RpcPromise constructor signature, so constructing
from a promised stub produces exactly the type a method returning that
stub would. The parameter accepts Promise<PayloadOrStub<T>>: the payload
itself or, for stubable payloads, a stub of it. NoInfer keeps the stub
arm out of inference, so an inferred T is always the promise's own
resolution type; stubs of non-stubable payloads are rejected because
ElideStub wouldn't elide them.

Type tests pin constructor/method equivalence for bare, callable,
plain-interface, and union payloads.

(Extracted from the pre-squash feat/result-stub-elision branch: the
constructor-alignment portion of its history that PR #253 didn't
include.)

* fix: align RpcPromise construction with result elision

Apply ElideStub -- the same transformation Result applies to a declared
stub return -- to the RpcPromise constructor signature, so constructing
from a promised stub produces exactly the type a method returning that
stub would. The parameter accepts Promise<PayloadOrStub<T>>: the payload
itself or, for stubable payloads, a stub of it. NoInfer keeps the stub
arm out of inference, so an inferred T is always the promise's own
resolution type; stubs of non-stubable payloads are rejected because
ElideStub wouldn't elide them.

Type tests pin constructor/method equivalence for bare, callable,
plain-interface, and union payloads.

(Extracted from the pre-squash feat/result-stub-elision branch: the
constructor-alignment portion of its history that PR #253 didn't
include.)

* fix: infer inline object literals with methods in the RpcPromise constructor

new RpcPromise(Promise.resolve({ x: 1, f() { return 1 } })) failed to
type-check: a literal with a method is context-sensitive, and the stub
arm of the constructor's Promise<PayloadOrStub<T>> parameter poisons
the contextual type, collapsing Promise.resolve's inference to never.
The same value predeclared in a const, or with an explicit type
argument, worked fine.

The constructor type is now an overload pair. The first overload takes
a plain Promise<T> -- no stub arm in the contextual type -- which is
the one context-sensitive arguments are typed against. Since
PayloadOrStub's stub arm is NoInfer anyway, both overloads infer
identically; the second (the previous signature) matters only when T
is explicitly annotated and the payload is a stub.

Type tests cover the inline literal, the predeclared equivalent,
explicit-type-argument-with-stub-payload, and a promise for a
target-or-stub union.

* chore: trim redundant type-test coverage and add the constructor changeset

- Drop the explicit-type-argument equivalence case from stub-elision
  (rpc-base-cases pins the same overload with a strict expectType) and
  the consumeMaybe(constructedMaybe) call already implied by the strict
  Equal against maybeViaMethod.
- Add a changeset for the constructor typing change; changeset-bot was
  flagging the PR as bump-less.

* test: pin RpcPromise constructor elision for ValidatedStub payloads

A ValidatedStub payload elides in the constructor (branded targets) or
keeps its stub type (plain interfaces) only because ValidatedStub
structurally matches capnweb's StubBase, which ElideStub keys on --
capnweb-validate maintains its own copy of that shape. Pin both cases
so drift in either package can't silently change the constructor's
type.

0.12.0 added a way to construct an `RpcPromise` from an ordinary `Promise`, so
callers can pipeline against a capability you have not obtained yet. That is
new public API, not a bug fix, and the docs said nothing about it -- main
documented it in the root README, which this branch replaced with a pointer to
here, so the merge would otherwise have dropped it on the floor.

`concepts/promises.md` gets a section, and it leans on the equivalence the API
docs make: wrapping a promise means the same thing as a local-loopback call
returning it. That is worth stating plainly, because it derives all the rules
that would otherwise have to be listed as a second set to memorise -- the
resolution is serialized, targets and functions come back as stubs, ownership
transfers, rejections propagate. The ownership one is the sharp edge and is
called out: disposing the promise disposes the resolution, so resolve with a
`.dup()` if you want to keep a stub.

The cheat sheet gets a one-line constructor note, matching the one `RpcStub`
already has.

Also: a Changelog entry in the sidebar under Reference, pointing at the GitHub
releases page. Off-site on purpose -- release notes are generated from
changesets on every publish, so a page here would be a copy that goes stale the
next time anyone ships. Nimbus recognises the absolute URL and adds
`target="_blank" rel="noopener"` itself.

Nothing else in the merge needed documenting. #241, #243, #251 and #253 are
leak and typing fixes with no API surface to describe; #253's note that
`RpcPromise<RpcStub<T>>` should now be written `RpcPromise<T>` applies to no
annotation anywhere in these pages.