fix(adapter): Cross-host collision detection, diagnostic locations, and declaration order

[Copilot]

Addresses feedback from PR #108 review thread. Fixes generator behavior when multiple hosts produce adapters with conflicting names, improves diagnostic precision, and preserves contract member ordering in generated code.

Cross-Host Collision Detection

Refactored pipeline to collect all hosts before generation, enabling conflict detection across the entire compilation:

// Previously: each host tracked conflicts independently (missed cross-host duplicates)
// Now: global tracking detects this scenario and reports PKADP006
[GenerateAdapter(..., AdapterTypeName = "SharedAdapter")]
public static partial class HostA { }

[GenerateAdapter(..., AdapterTypeName = "SharedAdapter")]  // ← PKADP006 reported
public static partial class HostB { }

Diagnostic Locations

• ValidateTargetMembers now reports diagnostics at member source locations instead of host location
• Falls back to host location only for metadata-only symbols
• Exceptions: overload detection remains at host location (aggregate diagnostic)

Declaration Order Preservation

• GetTargetMembers sorts by syntax declaration order (file path + line) when available
• Maintains contract member ordering in generated adapters for readability
• Metadata-only symbols use stable fallback (kind → name → signature → MetadataToken)

Documentation & Tests

• Clarified PKADP004 diagnostics are expected behavior when adapters share adaptee types, not false positives
• Added tests for PKADP018 (indexers) and cross-host PKADP006 scenarios
• Full diagnostic list (PKADP001-018) documented in AnalyzerReleases.Unshipped.md

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