v2: fix self-host chain

[medvednikov]

Summary

Fix the V2 self-host chain so v -o v2 v2.v && ./v2 -o v3 v2.v && ./v3 -o v4 v2.v completes reliably from cmd/v2.

The fix keeps flat-mode preregistration from decoding every function body up front, avoids unsafe mutation during generated str() function worklist iteration, avoids materializing the full expression-type list during cleanc alias collection, and prevents duplicate .vh inputs when cached headers are already present.

Root Cause

The second-generation V2 compiler could corrupt or exhaust runtime state while rebuilding itself:

• flat preregistration decoded full function bodies when only signatures were needed
• generated str() function discovery iterated a map that can be extended by recursive generation
• cleanc alias collection allocated a large temporary array of expression types
• cached .vh files could be appended twice in the flat/cached path

Validation

• ../../v -o v2 v2.v && ./v2 -o v3 v2.v && ./v3 -o v4 v2.v from cmd/v2
• isolated detached worktree at the PR commit with freshly built ./v, same self-host chain
• ./vnew -silent test vlib/v2/ast/ vlib/v2/types/checker_flat_test.v vlib/v2/builder/target_os_test.v vlib/v2/transformer/generated_fns_parts_from_flat_test.v
• git diff --check