When VERSION is set to a non-tag value like "main" (from goreleaser or
ldflags), the init() function unconditionally prepended "v", producing
"vmain". Now only add the "v" prefix when doing so produces a valid
semver string, leaving branch names and other non-semver values as-is.
Support both Unix epoch timestamps and RFC3339 format for build time
injection via ldflags. Unix epoch format provides simpler build
commands: $(date +%s) vs $(date -u +%Y-%m-%dT%H:%M:%SZ).
- Add parseBuildTime() to convert epoch to RFC3339
- Maintain backward compatibility with existing RFC3339 format
- Ensure consistent RFC3339 output regardless of input format
- Fix build date priority over git commit time
