A4: a fun interface method has no body

[milyin]

Fourth step of #6. Stacked on #9.

KtFunInterface::method was a full KtFun, which can carry a body, and
render_fun_interface rendered it verbatim — body included. A fun interface
must have exactly one abstract method, so that output had none at all:

fun interface Handler {
    fun run(v: Long): Boolean { return true }   // no abstract method -> error
}

KtFunSig is everything a KtFun has except a body and modifiers.
KtFunInterface::method holds one, so the shape is now unbuildable.

The type earns its keep twice

An abstract member is a signature, so KtFunSig is the natural way to
declare one:

KtClass::interface_("Codec").member(
    KtFunSig::new("encode")
        .param(KtParam::new("v", KtType::string()))
        .returns(KtType::byte_array()),
)

KtFunSig converts into KtFun (body None) and into KtDecl;
KtFun::signature() goes the other way. That last one replaces the
strip-the-body-and-modifiers helper a consumer would otherwise hand-write —
prebindgen-jni has exactly such a function today.

Rendering

KtFun and KtFunSig share one layout implementation through an internal
SigView, so the width-driven parameter breaking is factored out rather than
duplicated. Import collection splits too: a signature has no body, so only
parameter defaults can carry imports.

Verification

All 46 pre-existing tests pass untouched. Three added: the rendered
fun interface, a signature used as an interface member, and the
KtFun::signature() round trip. cargo clippy --all-targets clean.

[Copilot]

🟢 Ready to approve

The changes consistently enforce Kotlin SAM constraints, update rendering/import collection paths accordingly, and add targeted tests for the new behavior.

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Pull request overview

This PR strengthens the Kotlin declaration model by making fun interface (SAM) methods structurally unbuildable with bodies, introducing KtFunSig as a bodyless/modifierless function signature and reusing it for abstract-member rendering and import collection.

Changes:

• Introduces KtFunSig and switches KtFunInterface::method from KtFun to KtFunSig to prevent bodied SAM methods.
• Refactors rendering to share a single signature-layout implementation between KtFun and KtFunSig via SigView, and updates raw-import collection accordingly.
• Adds tests covering fun interface rendering, using KtFunSig as an interface member, and KtFun::signature() round-tripping.

File summaries

File	Description
src/model.rs	Adds KtFunSig, updates KtFunInterface to store a signature, and adds KtFun::signature() conversion.
src/render.rs	Adds shared signature rendering (SigView), adds render_fun_sig, and updates import collection for signature-only functions.
src/tests.rs	Updates raw-import collection test for SAM signatures and adds 3 new tests for the new API/behavior.
src/lib.rs	Re-exports KtFunSig as part of the public API surface.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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