docs(cli): correct break-even and region filter notes

[cristim]

Follow-ups from the review of #2197.

• docs/cli/filtering.md: break-even is infinite only when net savings are at or below -upfront/term; net 0 gives break-even equal to the term. --include/exclude-regions narrow only EC2 Instance Savings Plans; Compute, SageMaker and Database plans skip the region filter (shouldIncludeRecommendationRegion).
• resizeAfterExclusion: the error branch after common.ResizeCount was unreachable (newCount is max(0, Count-excluded) with excluded > 0, so it is always in [1, Count] here) and logged and dropped. It is now an explicit invariant panic, so a broken invariant cannot produce a count with unscaled money.

Tests: the existing extended-support adjustment tests and ScalesMoneyWithCount pass.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9806c9d and cd9c8ca.

📒 Files selected for processing: 2

• cmd/multi_service_engine_versions.go
• docs/cli/filtering.md

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[cristim]

Independent review (claude-opus-5-5) of cd9c8ca: APPROVE

No blocking findings.

Docs: both corrections match the code.

• Break-even: common.BreakEven returns +Inf only when EstimatedSavings + upfront/termMonths <= 0, and net 0 gives upfront / (upfront/term) = term.
• Region filter: cmd/multi_service_filters.go:128 returns true for region-agnostic plans before the include/exclude check.

Panic in resizeAfterExclusion: acceptable, non-blocking. I re-derived the invariant. The only caller is adjustRecommendationForExcludedVersions. It calls the function only when newCount = max(0, Count - excluded) with excluded > 0 and newCount != Count. So newCount >= 1 implies Count >= 2 and newCount <= Count - 1, and ResizeCount cannot refuse. The branch is unreachable.

Returning an error would need new plumbing through processRecommendation, which has no error return. The panic runs on the filtering goroutine before any purchase. It is not inside the recovered inventory workers (multi_service_engine_versions.go:127), so if it ever fired the run would stop before buying anything rather than continue with unscaled money.

Optional simplification: call common.ScaleRecommendationCosts(rec, float64(newCount)/float64(rec.Count)) and set Count. That removes the error, and with it the unreachable branch, entirely.

Evidence:

• go vet ./cmd is clean.
• Extended-support, exclusion and resize tests: 45 pass locally.
• All CI checks pass on this SHA, and CodeRabbit is clean.
• No runtime path changes except the unreachable branch, so I did not run anything live.