docs: update documentation for SPM support and testing framework detection

[ericodx]

Summary

Update all project documentation to reflect SPM support, testing framework auto-detection, the 6-stage discovery pipeline, and new execution components introduced in recent releases.

Architecture docs — Updated overview, discovery (6 stages including MutantIndexingStage and IncompatibleRewritingStage), execution (FallbackExecutor, TestResultResolver, SPMResultParser), configuration (nested RunnerConfiguration, ProjectType, TestingFramework, DetectedProject), and schematization docs.

CodeBase docs — Updated all 9 sections to reflect new types (ProcessRunner, ProcessRequest, SPMProcessLauncher, FallbackExecutor, TestResultResolver), updated signatures, and new data flow.

User-facing docs — README now covers SPM quick start and dual project support. USAGE.md adds SPM invocations, timeout defaults (120s Xcode / 30s SPM), and updated config reference. MUTATION-RESULTS.md adds SPM-specific performance notes.

Assets — Updated social preview subtitle.

Type of Change

• docs: Changes only to the documentation.

Invariants Checklist

• Original project is never modified — all mutations happen inside an isolated sandbox
• xcodebuild build-for-testing runs exactly once for all schematizable mutants
• No mutant results are lost or duplicated
• Mutant positions (file, line, column) are accurate in all reported results
• A cancelled task never leaves a simulator slot permanently acquired from the pool
• schematizedContent never contains the __swiftMutationTestingID global declaration
• Swift 6 Strict Concurrency compatible
• Pipeline stages remain stateless pure transformations

Pipeline Impact

• None

Testing

• All tests pass locally (swift test)