Harden unsafe unwraps, validate temporal bounds, and refresh README

[christopherkarani]

Motivation

• Remove risky force-unwraps and fragile runtime paths that can crash in edge conditions (GPU pipeline setup, generated CoreML wrapper, and temporal validation).
• Improve developer onboarding and first-run experience by surfacing a compact, actionable README quick-start.

Description

• Replaced a force-unwrapped raw pointer copy in MetalVectorEngine.search with a guarded withUnsafeBytes unwrap and removed a forced pipeline unwrap by selecting the active pipeline with explicit optional binding in Sources/WaxVectorSearch/MetalVectorEngine.swift.
• Replaced guard ... toMs! > ... style checks with safe if let validations for valid.toMs and system.toMs in Sources/WaxTextSearch/FTS5SearchEngine.swift to avoid force-unwrapping temporal bounds.
• Hardened generated CoreML wrapper Sources/WaxVectorSearchMiniLM/CoreML/all-MiniLM-L6-v2.swift by guarding the model output lookup and bundle URL lookup and emitting clear fatalError messages instead of force-unwrapping optional values.
• Rewrote and trimmed the top-level README.md to present a one-line purpose, a visible quick-start example, SPM install snippet, clear capability list, and when-to-use guidance for production onboarding.

Testing

• Attempted swift test --filter WaxCoreTests --filter WaxIntegrationTests/READMEExamplesTests, but the invocation failed early due to an existing package manifest issue: error: 'wax': invalid custom path 'Tests/WaxTests' for target 'waxTests' (this is a pre-existing project config problem and blocked running the full test suite).
• Per-file static safety checks were performed (grep/inspection) on the modified files to confirm the removed force-unwrap patterns; those checks passed for the updated files.
• Changes were committed locally: the patch includes edits to MetalVectorEngine.swift, FTS5SearchEngine.swift, all-MiniLM-L6-v2.swift, and README.md.

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Codex Task

[christopherkarani]

@codex review and fix
@claude review and fix

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