Guard and harden mapget releases

[josephbirkner]

Summary

The v2026.3.5 release initially reached the deploy job with CMake reporting 2026.3.4 while setuptools_scm reported 2026.3.5. All wheels had already been built by the time the mismatch was detected.

This change makes that failure mode impossible in the normal release path:

• validate the CMake, SCM, release-branch, and expected release versions before starting the build matrix
• add focused unit coverage for release-version validation
• add a guarded Create Release workflow which tags the already-green main commit and dispatches deployment explicitly
• make CI and Deploy manually dispatchable for controlled recovery
• disable sccache for release builds to avoid external cache outages blocking publication
• disable matrix fail-fast so one transient platform failure does not cancel all sibling artifacts
• document the release procedure and discourage creating tags from GitHub's release form

Validation

• python3 .github/scripts/test_validate_version.py -v
• python3 -m py_compile .github/scripts/validate_version.py .github/scripts/test_validate_version.py
• focused tag, branch, and explicit-version validation contexts
• git diff --check
• YAML parsing with PyYAML
• actionlint v1.7.12

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Package	Line Rate	Branch Rate	Health
libs.geojsonsource.include.geojsonsource	100%	50%	✔
libs.geojsonsource.src	66%	38%	➖
libs.gridsource.include.gridsource	0%	0%	❌
libs.gridsource.src	0%	0%	❌
libs.http-datasource.src	48%	26%	❌
libs.http-service.src	58%	24%	➖
libs.location.include.mapget.location	100%	100%	✔
libs.location.src	83%	40%	✔
libs.logging.include.mapget	5%	2%	❌
libs.model.include.mapget.model	51%	30%	➖
libs.model.src	73%	42%	➖
libs.service.include.mapget.service	56%	50%	➖
libs.service.src	74%	38%	➖
Summary	65% (12142 / 18562)	35% (12920 / 37286)	➖

Minimum allowed line rate is 50%