Improve README documentation clarity

[claudeaceae]

Summary

This PR improves the README documentation to address potential confusion and enhance clarity for developers:

• Explains v26 versioning: Adds a note explaining Apple's unified versioning system (v26) to prevent confusion about the version jump from v18 to v26
• Clarifies scope: .ancestor: Adds clearer explanation and inline comment for when/why to use the ancestor scope parameter
• Updates library author guidance: Modernizes the version range example to be more current (25.0.0..<27.0.0 instead of 1.3.0..<27.0.0)
• Fixes minor inconsistencies: Corrects hyphenation (opt-in, future-proof) for consistency

Motivation

While reviewing the README, I noticed that the version jump from .v18 to .v26 could be confusing to developers unfamiliar with Apple's 2025 unified versioning system. The scope parameter explanation was also a bit terse and could benefit from additional context.

Changes

1. Added a NOTE callout explaining the v26 unified versioning system right after the IMPORTANT callout about explicit opt-in
2. Enhanced the scope parameter section with a bold heading and inline code comment
3. Updated the library author version range example to use a more realistic current version range
4. Minor grammar/style improvements (hyphenation consistency)

Test plan

• Read through the updated README
• Verified all markdown formatting renders correctly
• Confirmed links still work
• No code changes, documentation only

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