feat(enum): implement a trait without passing its companion

[jo16oh]

Why

Val accepts a trait as a type argument when the trait implements nothing of its own:

const Row = Val.companion<Row>().implTrait<Wired>({ toWire });

An enum could not. The call failed with TS2558: Expected 2 type arguments, but got 1, and the
second argument lost its contextual type along with it. Nothing records the omission as a decision:
notes/design.md §15.2 documents only the companion form, and §9 has no entry.

src/enum.ts also carried its own thinner Takes and Passes, so five gates Val applies never
reached an enum: the mis-call message, a member name another trait answers to, a member name a
variant's field holds, Complete, and Alone.

What changed

• Val's Takes, Complete, Passes and Alone are shared instead of copied. enum.ts drops
  its own, and both entry points publish the same two overloads Val does.
• PayloadKeys distributes. keyof over the union of an enum's payloads keeps the shared fields
  alone, so a member shadowing one variant's field passed the gate.
• The companion's position may hold the members themselves: trait.__valof_shared ?? trait, one
  token, matching val.ts.
• api.md lists the type-argument form, and marks the arguments that were already optional.

Cost

The declarations shrink: the two duplicated aliases outweigh the overloads doubling across the two
entry points. §15.1 paid +2.1 kB for the same overloads on a Val.

	before	after	budget
index.d.mts	58.81 kB	58.67 kB	64.00 kB
enum instantiations	32,238	32,646	90,000
core instantiations	5,733	5,733	10,000
production gzip, with Enum	106 B left	105 B left	1.38 kB

Tests

Each gate was removed from the source to confirm a test goes red, on both entry points: the sealer's
and the companion's. EnumBuilder had no coverage at first, and losing every gate left the suite
green.

One behaviour to note

Two gates now reject code that compiled before: a trait member shadowing one variant's field, and
two traits answering to the same member name. Both were holes rather than allowances, and Enum
ships from valof/experimental, so this is labelled as a fix rather than breaking. Say if it
belongs in the release notes as one.