refactor!: drop the class-level ~standard, leaving instance as the entry point

[btravers]

// before
fromSchema(Organization);
// after
fromSchema(Organization.instance);

What it bought, measured

fromSchema(Person)          → Person
fromSchema(Person.instance) → Person        ← identical
instance has ~standard?     → true          ← zod v4 provides it already

Exactly one thing: letting you drop .instance. instance is a zod schema and
zod implements Standard Schema, so it already carries ~standard and is
accepted by anything that takes one.

What it cost

The class was a validator in some contexts and not others. The README had
to say so:

It does not make z.object({ owner: Organization }) work — zod requires a
real ZodType, so nesting always goes through Organization.instance.

Two spellings of one concept that are not interchangeable, and you had to know
which context takes which. That is the same shape of problem that removed
encode() and collapsed decode/make.

#14 sharpened it further: .instance is now a valid entity field, so it is
unambiguously the composable surface. The class-level shortcut was the odd one
out.

Changes

• attachInstance loses its second accessor; its comment no longer has to
  explain rebuilding "the schema (and its ~standard)"
• EntityStatic loses the member
• instance.spec.ts keeps the same coverage against instance, and gains an
  explicit assertion that instance is a Standard Schema — which is the fact
  this change relies on
• README, package README and CLAUDE.md updated; CLAUDE.md also gains the
  union.ts entry it was missing from feat: add union(), a union of entities that is itself entity-like #15

Gate

format --check, lint, typecheck (three passes), test (108, 10 files),
knip, build — all green.

[Copilot]

Pull request overview

This PR removes the entity class-level Standard Schema entry point (~standard) so that Entity.instance is the sole composable/Standard-Schema surface, aligning with how zod v4 already implements Standard Schema on schemas and avoiding having two non-interchangeable spellings of “the validator”.

Changes:

• Dropped ~standard from the entity static type surface (EntityStatic) and from runtime attachment (attachInstance).
• Updated docs and examples to consistently use fromSchema(Organization.instance).
• Updated tests to assert instance remains the Standard Schema entry point and that instance stays non-enumerable and cached.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Updates Standard Schema guidance/examples to route through Organization.instance.
packages/entity/src/types.ts	Removes ~standard from the EntityStatic type surface.
packages/entity/src/instance.ts	Removes runtime definition of class-level ~standard; keeps lazy, cached instance.
packages/entity/src/instance.spec.ts	Adjusts tests to validate instance as Standard Schema entry point and removes class-level ~standard expectations.
packages/entity/README.md	Removes ~standard from the documented statics table; reinforces instance as the composable surface.
CLAUDE.md	Updates module documentation for instance.ts and adds the missing union.ts entry.
.changeset/drop-class-standard-schema.md	Adds a breaking-change note and migration guidance (fromSchema(X) → fromSchema(X.instance)).

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