refactor: replace checklistRunner cache triples with a generic lazycache

[youngjinjung-linq]

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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[YoungJinJung]

Recreated as #261 from an in-repo branch under the maintainer account; this PR is superseded.