A2: a companion object becomes its own type

[milyin]

Second step of #6. Stacked on #7.

KtClassKind::Companion was documented as "only valid as KtClass::companion"
and nothing enforced it. A companion object could be built as a top-level
declaration, and the renderer would emit companion object { … } at file level,
where it means nothing.

KtCompanion is now its own type with no impl Into<KtDecl>, so it cannot
reach a declaration position at all. That is proved by a compile_fail doctest
rather than by a runtime check:

/// ```compile_fail
/// use kotlin_codegen::{KtCompanion, KtFile};
/// let _ = KtFile::new("io.p").decl(KtCompanion::new());
/// ```

Being a distinct type, it also has no KtClassKind to be handed the wrong
variant and no primary constructor to misuse — a companion object cannot have
one, and now there is nowhere to put one.

Incidental cleanups

• name: Option<String> replaces the empty-string-means-anonymous sentinel
  the renderer used to special-case. Naming a companion (companion object Factory) is now explicit instead of implied by a non-empty string.
• KtClass::companion drops its Box — the indirection now comes from the
  Vec<KtDecl> inside KtCompanion.
• Supertype rendering is shared between class and companion instead of
  duplicated.

Verification

All 41 pre-existing tests pass untouched — output is byte-identical. Two
tests added for the named and anonymous forms, plus the compile_fail doctest.
cargo clippy --all-targets clean.

[Copilot]

🟡 Changes recommended

KtCompanion::named currently permits empty names, which can render invalid Kotlin (companion object ) and should be rejected or normalized.

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Pull request overview

This PR is the second step of the validation umbrella work, restructuring the declaration model so a Kotlin companion object cannot be constructed in an invalid top-level declaration position by making it a dedicated KtCompanion type.

Changes:

• Introduces KtCompanion as its own model type (no Into<KtDecl>), and updates KtClass to hold Option<KtCompanion> instead of a boxed KtClass.
• Updates the renderer to render companions via a dedicated render_companion path and shares superclass/interface rendering via render_supertypes.
• Updates and extends tests to cover named vs anonymous companion rendering.

File summaries

File	Description
src/model.rs	Removes KtClassKind::Companion, adds KtCompanion, and updates KtClass companion storage/API accordingly.
src/render.rs	Adds companion-specific import collection/rendering and factors supertype rendering into a shared helper.
src/tests.rs	Migrates companion construction to KtCompanion and adds coverage for named/anonymous rendering.
src/lib.rs	Exports KtCompanion from the crate prelude.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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