[CFG] CFG edge validator#703
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The endpoint-only invariants (start_node / final_node in range, both non-null) don't catch the asymmetric-edge corruption typical of a pre-CFG IR transform producing malformed IR that build_cfg then turns into a CFG with mismatched prev/next pointers. Such corruption surfaces as a segfault deep in worklist propagation, not at the boundary. Add a second phase to assert_structural_invariants: for every node n and every successor m in n->next, require m->prev to contain n (and symmetrically for predecessors). Null entries are tolerated -- erase() clears entries before simplify_graph compacts, so the intermediate non-compact state must still validate. Cost is O(V+E), small for typical kernels and dominated by the analyses that follow; left always-on (release builds too) because the alternative on violation is silent corruption.
When a structural-invariant check fires, the assert message now points at a fresh file under <tmp>/cfg_invariant_failures/structural_<ns>.txt containing the CFG state at the moment of violation -- node-by-node ranges, prev/next edges, intra-node next/prev links, the offending nodes' statements. Designed for post-mortem on the kind of failure that shows up after a pre-CFG IR transform (e.g. an unstructured-control-flow normaliser) produces a CFG that build_cfg cannot canonicalise: instead of "assert fired, now bisect from scratch" the user has a viewable graph at the exact failure point. The dumper is deliberately tolerant of malformed state. It is invoked only after a violation has been detected so the graph is by construction not internally consistent: null entries print as <ERASED>, per-node dump calls are individually try/catch wrapped so a single bad pointer doesn't take down the whole dump, and the outer try/catch handles filesystem failures. Returns the path on success or empty if the dump itself failed; the assert message picks "<dump failed>" in that case. Also fold the previously-one-per-violation QD_ASSERT_INFO calls into a single comprehensive message at the end -- the endpoint phase now collects all violations before bailing, so the user sees every broken endpoint at once instead of fixing them serially. Edge-consistency phase still short-circuits on first violation (subsequent edges may share corrupted pointers).
💡 Codex Review Replace C++20 designated initializers
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