v2: finish cleanc flat AST self-host migration (#27390)

Author: medvednikov

vlib/v2/README.md

@@ -10,4 +10,4 @@ parsing, type checking, markused, and native SSA input:
 - `ast.flatten_files(files)` builds `ast.FlatAst` with contiguous `nodes` and `edges`.
 - `ast.legacy_ast_stats(files)` estimates memory/shape metrics for recursive AST files.
 - `flat.stats()` and `flat.count_reachable_nodes()` report flat graph size and reachability.
-- `V2_LEGACY_AST=1` keeps the old recursive-AST pipeline available for comparison.
+- `flat.to_files()` remains available for focused flat-reader roundtrip tests.