[Grove] Fix peer cyclic module dependency detection

[MonzerMansour]

Summary

• Detect required peer dependency cycles (both modules in Configuration) that the search-stack algorithm missed.
• Add a buildTypeOrder DFS safety net so optional/mixed peer cycles error with searchStackCycle instead of infinite recursion.
• Document that mutual peer required dependencies also fail at resolve time.
• Expand DependencyTests for peer, longer, optional, mixed, acyclic, and diamond graphs.

Motivation

Issue #18’s reproduction (ModuleA ↔ ModuleB both configured) crashed in recursive type-order DFS because require early-returns for already-queued peers and never extends the search stack.

Changes

Area	Change
DependencyManager.swift	Required-edge cycle detection + DFS visited/in-progress safety net
Module-Dependency.md	Clarify peer mutual deps error at resolve
DependencyTests.swift	Regression coverage for #18 cases

Test plan

• xcodebuild test -scheme Grove-Tests -testPlan Grove -destination 'platform=macOS' → TEST SUCCEEDED (62 tests)
• peerModuleCycle, longerPeerModuleCycle, optional/mixed peer cycle tests green
• Existing moduleCircle1 + acyclic/diamond cases green
• CI green on this PR

Out of scope

• Officially supporting cyclic dependencies (including treating optional cycles as valid)
• Unrelated monorepo packages

Closes #18

[coderabbitai]

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