refactor!: remove encode(), leaving toJSON() as the only projection

[btravers]

Supersedes the original rename approach. Per review, the method is removed
rather than renamed — the naming collision it was meant to fix disappears when
the method does not exist.

Why removal beats renaming

encode() and toJSON() returned byte-identical data:

spread     : {"id":"…","cachedSummary":"leak me"}
encode()   : {"id":"…"}
toJSON()   : {"id":"…"}

Two public methods, one value — the alias CONTRIBUTING.md exists to prevent.

toJSON() is the one that has to stay. It is not a name this package chose; it
is the hook JSON.stringify looks for. Without it, serializing an entity walks
own enumerable properties and leaks a subclass's own instance fields — the same
bug spread has, as the first line above shows. So encode() was the removable
one.

It was also genuinely misnamed: it returned the stored (decoded) shape
while the exported Encoded<T> helper names the wire shape. For an entity
using decoded: { omit, add } these differ — Encoded<typeof ApiKey> carries
secret, the projection carries fingerprint and no secret. That mismatch is
why decode(x.encode()) never round-tripped, and the README now frames the
pairing positively (make(x.toJSON())) instead of apologising for a name.

Changes

• entity.ts — encode() deleted. toJSON, equals and update now route
  through a module-private project, so the internals do not depend on a
  serialization hook a subclass is free to override.
• types.ts — encode() removed from BaseInstance; the DeepReadonly
  comment updated to reference the one remaining projection.
• Specs, entity.test-d.ts, both READMEs, CLAUDE.md — all call sites and
  prose updated (38 references).
• entity.test-d.ts gains a @ts-expect-error pinning that encode() is gone.
  That assertion passing tsc is the proof the removal is complete.
• CLAUDE.md corrected while here: it said "Five source modules" and referenced
  encode(); freeze.ts from fix: make entity data deeply immutable #7 makes six.

Also: entityName documented

Finding 5's second half. It was tested but documented nowhere in the root
README. Added, with the reason all three tag spellings coexist without being
aliases: _tag is instance-side and what P.tag(...) matches; entityName is
the only path for code holding the class and no instance; both derive from the
single Entity(tag) declaration.

Gate

format --check, lint, typecheck (both passes), test (76, 9 files),
knip, build — all green. Rebased onto current main, so the earlier
conflict with #7 is gone.

[Copilot]

Pull request overview

This PR resolves an API naming mismatch in the entity instance surface by renaming the instance method encode() (which returned the stored/decoded shape) to toStored(), aligning the name with the data shape it actually returns and the make() round-trip it supports.

Changes:

• Renamed the instance method encode() to toStored() across the implementation and type surface.
• Updated internal uses (toJSON(), equals(), update()) to delegate to toStored().
• Updated documentation, specs, and added root README documentation for the existing entityName static.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Replaced encode() references with toStored(), reframed round-trip guidance, and documented entityName usage.
packages/entity/src/types.ts	Updated the shared instance interface surface from encode() to toStored().
packages/entity/src/equality.spec.ts	Updated equality test to use toStored() instead of encode().
packages/entity/src/entity.ts	Renamed the instance method implementation to toStored() and updated internal call sites (toJSON, equals, update).
packages/entity/src/entity.spec.ts	Updated instance-method behavior tests to reflect toStored().
packages/entity/src/decoded.spec.ts	Updated decoded/omitted/add behavior tests and round-trip test wording to use toStored().
packages/entity/README.md	Updated package README method docs from encode() to toStored() and clarified entityName as _tag’s static counterpart.
.changeset/silly-donuts-smoke.md	Added a breaking-change changeset describing the rename and migration.

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