Prepare pjson 1.0.0 release

[jdpraveen]

Summary

• freeze the accumulated changelog as pjson 1.0.0
• adopt the requested plain Semantic Versioning tag 1.0.0
• update release, versioning, security, and README status documentation
• publish a SHA-256 checksum alongside the generated API-reference archive

Verification

• ./build.sh --all --auto
• 431/431 registered tests passed
• ASan/UBSan, bounded libFuzzer replay, JSONTestSuite, and supported JSON Schema draft-07 corpus passed
• static/shared relocatable CMake and pkg-config package consumers passed
• formatting, clang-tidy, documentation validation, and REUSE licensing passed
• current main CI, Documentation, and CodeQL workflows are green
• no open CodeQL alerts, Dependabot alerts, or private security advisories

Release

After this PR is approved and squash-merged, create the annotated 1.0.0 tag from the exact merge commit and publish the GitHub release titled pjson 1.0.0.