docs: add framework audit, CONTRIBUTING, and modernize README metadata

[christopherkarani]

Motivation

• Align public-facing docs with the package manifest and repository reality by correcting platform/version and license metadata in README.md.
• Provide an explicit contributor on-ramp and expectations by adding a CONTRIBUTING.md so new contributors and CI know how to run tests and open PRs.
• Deliver a comprehensive framework audit that inventories risks, concurrency/Sendable concerns, API ergonomics, and prioritized next steps for a TDD-led cleanup.

Description

• Rewrote README.md to a compact, accurate quick-start covering SPM install, supported platforms (iOS 18 / macOS 15), core API shape, and a short “when to use / when not to use” section.
• Added CONTRIBUTING.md that documents local development, test invocation (swift test), PR expectations, and reporting guidance.
• Added docs/framework-audit-2026-03-06.md containing the full audit (summary, bug table, type-system & concurrency recommendations, naming/namespace remarks, and a TDD-focused next-step plan).

Testing

• Ran the README-related test slice via swift test --filter READMEExamplesTests, which failed with an unrelated package layout error: error: 'wax': invalid custom path 'Tests/WaxTests' for target 'waxTests'.
• No other automated test failures were introduced by these documentation additions, and the change is documentation-only (no source API breaking changes were made in this PR).

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Codex Task