Versioning: stages v0.1, firmware SemVer, software CalVer

TEMPLATE.md Versioning section; notes in README, CHANGELOG and the firmware/software READMEs. Source: contrib/product-template in B4L/proplan.

software/README.md

@@ -3,3 +3,8 @@
 Host tools and apps (diagnostics, configuration, test rigs), one folder per
 tool, for example `software/diag/` with its own README and pyproject.toml or
 Cargo.toml.
+
+Versions: CalVer per tool, tagged `software/<tool>/YYYY.MM.MICRO` (for
+example `software/diag/2026.10.0`: month without a leading zero, MICRO
+counting that month's releases from 0). Each tool folder has its own
+`CHANGELOG.md`, and its README says which firmware MAJOR versions it talks to.