Add series retrospective to the MLIRGen refactoring review — §1 complete

[ASDAlexander77]

Summary

Docs only. Closes out docs/MLIRGen-refactoring-review.md with a retrospective of the #201–#227 series:

• Final architecture map: the 26,702-line monolith is now 13 TUs (driver + private header + 11 family satellites), with the header at ~10.7k lines holding templates, RAII scopes, and small helpers.
• Latent bugs the series surfaced and fixed — nested-discovery clobbering, caller-context flag/type leaks, the mapped-type key leak, the inference non-convergence with its load-bearing double-count, dangling FuncOp handles, GenContext init/ownership hazards.
• The extraction recipe with its hard-won edge cases (whitespace-tolerant brace matching, K&R signatures, template look-behind, default-arg handling, multiset reconciliation, full-suite gating).
• Open items: the A7 upward-mailbox channel redesign (awaiting review) and the optional header tail polish.

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