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

[milyin]

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

This PR is not intended to be merged. It preserves the typed-expression
AST / expr module that was removed from main in #2, so the work is easy to
find, review, and revive later.

Why a separate archive PR

The AST was part of the codebase since the initial import, so a PR from the
original pre-cleanup branch (backup/before-cleanup) shows an almost-empty
diff — GitHub diffs against the merge-base, which already contained the AST.
To make the infrastructure actually visible as a diff, this branch restores
those files on top of the current (AST-free) main, yielding a clean additive
diff of ~3.6k lines.

What's here

Restores the full typed-expression layer:

• src/expr.rs, src/expr/render.rs, src/expr/tests.rs — the AST model,
  renderer, and its test suite
• Pre-cleanup versions of model.rs, slot.rs, render.rs, lib.rs,
  file.rs, code.rs, and tests that integrated the AST

Key types being preserved: ExprArena, KtExpr, KtStmt, KtLambda,
KtLiteral, KtPattern, KtName, BindingId/ArenaId, Binder,
ExprSlot<T>, Ast<T>, AnnotationSlot/KtAnnotation, KtAccessor, and
KtFun::typed_body.

Caveats (archive, not mergeable)

• It reverts the Kt-prefix renames and the overload-validation change that
  now live on main for the affected files. Reviving this work means
  re-integrating the expr module on top of current main, not merging as-is.
• Tests/mergeability were intentionally not reconciled.

Raw pre-cleanup source is also preserved on branch backup/before-cleanup.

Companion to #2 (the cleanup that removed this infrastructure).