[ARCHIVE] Typed-expression AST infrastructure (saved for future revival)

[milyin]

Purpose — preservation, not merge

This PR archives the typed-expression AST infrastructure so it is not lost when PR #2 (cleanup/remove-unused-ast-infrastructure) deletes it from main.

It is intentionally not meant to be merged as-is — mergeability and CI are not a concern. It exists as a durable, reviewable reference so the AST work can be revived later when an emitter is ready to build typed trees instead of raw KtCode strings.

Note: while PR #2 is still open, the diff below is small (main currently still contains the AST). Once PR #2 merges into main, this PR's diff will automatically expand to show the full AST infrastructure as a re-addable set of changes.

What this branch preserves

The complete expr module and its integration points:

• ExprArena — ownership/provenance tracker for expression binders. Allocates BindingIds per-arena; graft alpha-remaps when composing trees from different arenas, making variable capture structurally impossible.
• KtExpr / KtStmt / KtLambda / KtLiteral / KtPattern — the typed Kotlin expression/statement AST.
• BindingId / ArenaId / Binder / Spelling / NameHint — binder identity and rendering-name machinery (Fixed for API-surface parameter names, Fresh for renderer-allocated locals).
• Tree operations — fill_hole, substitute, free_names, has_hole.
• Slot bridges — ExprSlot<T>, Ast<T>, AnnotationSlot, KtAnnotation, StaticAnnotationText, KtAccessor, AccessorTree, and KtFun::typed_body — the exclusive Legacy(text) vs Ast(tree) sums that let declarations carry either raw text or typed trees.
• The full internal test suite for the above (arena provenance, alpha-renaming, capture-avoidance, etc.).

Why it was removed from the mainline

No emitter (prebindgen-jni or any other consumer) constructs typed trees yet — the entire tier was #[allow(dead_code)] infrastructure staged ahead of its consumers. PR #2 removes it to keep the shipping public API minimal. This archive keeps the design and its tests recoverable.

Also included

• Commit 598e19d — validate function overloads by parameter-type lists in merge_files (independent of the AST work).

How to revive

Branch from here (or cherry-pick the expr module back) once a code generator is ready to emit typed expression trees, then re-wire the ExprSlot/AnnotationSlot/KtAccessor bridges into the model.

[milyin]

Superseded by #4. This branch's PR diff didn't show the AST because GitHub diffs against the merge-base, which already contained it. #4 restores the AST on top of current (AST-free) main so the full infrastructure is visible as an additive diff. The raw pre-cleanup source remains preserved on branch backup/before-cleanup.