v2: finish flat AST migration

[medvednikov]

Summary

• make the v2 flat AST pipeline the default path for parsing, checking, transforming, SSA, and CleanC backend flow
• migrate transformer passes to operate from flat AST cursors/readers without rehydrating whole files
• add flat-aware handling for imports, comptime import conditions, type propagation, monomorphization, live reload injection, and runtime init
• fix compiler-side lowering issues for map or {} expressions and raw array for in codegen so source-level avoids are no longer needed

Root Cause / Impact

The v2 pipeline still had several places that decoded or depended on rehydrated AST files, which blocked using flat AST consistently. During self-hosting, that also exposed compiler bugs around map-index or {} lowering and typed runtime array iteration; those are fixed in compiler code instead of avoided in source.

Validation

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew -silent test vlib/v2/gen/cleanc/cleanc_test.v
• ./vnew -silent test vlib/v2/transformer/transformer_flat_diff_test.v
• ./vnew -silent test vlib/v2/ssa/optimize/
• ./vnew -silent test vlib/v2/transformer/
• ./vnew -silent test vlib/v2/builder/gen_cleanc_parallel_test.v
• ./vnew -silent test vlib/v2/gen/cleanc/generic_struct_scan_test.v
• git diff --check
• cd cmd/v2 && ../../v -o v2 v2.v && ./v2 -o v3 v2.v