Prepare 0.3.0-alpha release

[Aaronontheweb]

Summary

• set the package version to 0.3.0-alpha for Netclaw integration
• add release notes for the structured syntax, occurrence, value, and redirect APIs
• correct all release instructions to use the bare SemVer tag that the publish workflow accepts
• make release-note extraction select the exact pushed version and fail when that section is absent

Validation

• Release build: clean, zero warnings
• Tests: 2,370 passed
• Package: ShellSyntaxTree.0.3.0-alpha nupkg and snupkg
• Public API comparison against 0.2.0: no breaking changes
• OpenSpec strict validation: passed
• Header verification: passed
• Slopwatch: zero findings
• Adversarial review: GO on immutable staged hash 98013132f80c6a5fe42adad5877b74ebfa7bd2b26f90ddc1704c5aee5a2340bd

After this PR merges, the bare 0.3.0-alpha tag will start the NuGet publish workflow.