A5: external is a body kind, not a modifier

[milyin]

Fifth and last structural step of #6. Stacked on #10.

external was a free-form string in KtFun::modifiers while the body lived in
a separate field, so this was representable and rendered:

external fun f(): Int { return 1 }   // natively implemented *and* has a body

Making it a KtBody variant puts both in the same field, so setting either
clears the other and the combination has nowhere to exist:

KtFun::new("f").external().body(code)   // -> Block
KtFun::new("g").body(code).external()   // -> External

KtFun::external() replaces .modifier("external"), and passing the string
panics with a message pointing at it — otherwise the old spelling would quietly
reintroduce the hole this PR closes.

What stays for the validator

KtBody::None remains. Whether a bodiless function is legal is positional —
an interface member needs no keyword, an abstract class member does — and
position is not something a type can capture here. It is listed as B3 in the
umbrella.

Verification

All 49 pre-existing tests pass; external renders in exactly the same position
as before, so no golden string moved. Three added: rendering, the
mutual-exclusion property, and the modifier-string panic.
cargo clippy --all-targets clean.

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Part A is complete with this PR. Five shapes that used to render as
non-compiling Kotlin are now unbuildable: constructor parameters on kinds that
have no constructor, empty data class, wrong-arity value class, top-level
companion object, two constructed superclasses, a bodied fun interface
method, and external with a body.

[Copilot]

Pull request overview

Moves Kotlin external from a free-form function modifier string into the typed function body representation (KtBody::External) so external fun … { … } becomes unrepresentable in the model, while keeping rendering order consistent and adding regression tests.

Changes:

• Add KtBody::External and a dedicated KtFun::external() builder; reject "external" passed via KtFun::modifier(...).
• Update rendering/import-collection logic to treat External like a bodiless function while still rendering the external keyword ahead of other modifiers.
• Update and add tests covering rendering, mutual exclusion of external vs body, and the modifier-string panic.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/model.rs	Introduces KtBody::External, adds KtFun::external(), and guards against "external" being added via modifier().
src/render.rs	Renders external from the body-kind and treats External like None for body output and body-import collection.
src/tests.rs	Migrates existing tests to .external() and adds new tests for rendering, exclusivity, and the panic guard.

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