chore: rename internal/saturation to internal/saturationv1

[mamy-CS]

Relates to #1402 (Section 4 of 7: resolve the saturation package-name collision)

Proposed Changes

• 🧹 Rename package internal/saturation → internal/saturationv1 (3 files moved, byte-identical except the package declaration)
• 🧹 Update the 3 importers (collector/replica_metrics.go, engines/pipeline/enforcer.go, engines/saturation/engine.go): import paths + 6 saturation.X → saturationv1.X references
• 🧹 Point the V1 deprecation note in engine.go at its tracking issue (Track: deprecate & remove the V1 (percentage-based) saturation analyzer #1441) instead of a vague standalone line

Why

internal/saturation (V1 analyzer) and internal/engines/saturation (the Engine) were both package saturation. Because engine.go (itself package saturation) imported the V1 package unaliased, saturation.NewAnalyzer() in that file resolved to the imported package; easy to misread. After this rename, the only remaining package saturation is the Engine, so the ambiguity is gone.

V1 is deliberately not moved under internal/engines/analyzers/ alongside throughput/queueing-model/saturation-v2: unlike those, V1 does not implement the Analyzer interface (its surface is AnalyzeModelSaturation / CalculateSaturationTargets), so grouping it there would imply a peer relationship that doesn't exist. It stays the default analyzer path; eventual removal is tracked in #1441.

Pre-review Checklist

• E2E tests for any new behavior — N/A, pure rename, no behavior change
• Docs PR for any user-facing impact — N/A, internal-only, no user-facing surface touched
• Proposal PR for any new enhancement or change to existing behavior — N/A, no behavior change

Release Note

NONE

Docs

None needed. V1 removal is tracked separately in #1441.

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Verification for reviewers: git tracks all 3 as renames (history preserved); the moved files are byte-identical to their originals except the package line; the only remaining package saturation is internal/engines/saturation; no stale internal/saturation references anywhere. go build, go vet, gofmt -l (clean), make lint (0 issues), and the full non-e2e go test suite all pass.

[asm582]

@mamy-CS saturationv1 is no longer the default; this code should be removed.

[mamy-CS]

Collaborator

Yes, I was going to make it easier for the removal by explicitly renaming it. The v1 removal tracker is here, #1441) Although v1 is still going to stay selectable in v0.9 per the issue tracker.