refactor: replace checklistRunner cache triples with a generic lazycache

[YoungJinJung]

Summary

Implements #204: checklistRunner repeated the same xLoaded bool / xErr error / xBy... map pattern seven times (RDS, hosted zones, VPCs, subnets, log groups, security groups, secrets).

• New lazyCache[T] memoizes a single fetch — first Get runs the fetch once, later Gets return the same value and error without refetching (including memoized failures, matching the old behavior).
• Each resource group's maps move into a small index struct (vpcIndex, subnetIndex, …) built inside the fetch closure.
• Lookup logic (ID/name/ARN resolution, ambiguity errors, VPC scoping) is unchanged; the subnet loader still composes on the VPC loader through the cache.
• secretDetails and zoneRecords stay as plain per-key maps — they are keyed caches, not load-once lists.

Testing

• go test ./... passes; existing checklist tests cover the resolution behavior, and a new test pins the lazyCache contract (fetch-once, memoized value, memoized error, no retry).
• make build passes.

Closes #204

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📥 Commits

Reviewing files that changed from the base of the PR and between 744b139 and ddc81a3.

📒 Files selected for processing (2)

• internal/inspector/checklist.go
• internal/inspector/checklist_lazycache_test.go

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[amazon-q-developer]

This refactoring successfully eliminates code duplication by introducing a generic lazyCache[T] pattern. The implementation correctly memoizes both successful values and errors, maintaining the same behavior as the original code. All existing tests pass, and new tests validate the cache contract. No defects found that would block merge.

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[youngjinjung-linq]

과설계 관점 리뷰: 이 refactor는 반복 필드를 줄이지만 production diff가 오히려 24줄 순증가하고 일회성 타입 7개를 추가합니다.

[youngjinjung-linq]

[P3] yagni: generic lazyCache 하나를 넣기 위해 일회성 index wrapper 7개와 각 Get closure를 추가해 production code가 net +24 lines가 됐습니다. 기존 runner-local loaded/error/map triples가 못 해결하던 두 번째 사용처나 동시성 요구도 없습니다. 이 변경은 추상화 비용이 절감보다 큽니다; 적어도 wrapper types 없이 각 loader가 필요한 map 묶음을 직접 캐시하게 줄여 주세요. net: -24 lines possible.

[YoungJinJung]

Addressed: dropped the generic lazyCache[T] and all seven index wrapper types. The per-resource maps are direct runner fields again (as on main); only the load-once bookkeeping is factored into a 12-line loadState whose do() memoizes both success and failure. The checklist file is now a net -8 production lines vs main instead of +24, and the seven loaded/err pairs are still gone.