fix: CalVer not working correctly with zero-padded formats (fixes #320)

[oharo3109]

Problem

When using CalVer formats with zero-padded components (e.g., YYYY.0M0D.MICRO), tagpr was
generating PRs with the same version as the current one instead of the expected new CalVer
date-based version.

Example: With format YYYY.0M0D.MICRO and current tag v2026.0227.10, running tagpr
on March 2nd should produce v2026.0302.0, but instead it produced v2026.0227.10.

Changes

• semver.go: Naked() falls back to originalVersion when asCalendarVersion=true
  and cv==nil, preserving zero-padded components like 0M0D.
• tagpr.go: After cherry-picks, CalVer mode re-applies bumpVersionFile instead of
  reading from the file to override nextVer.
• calver_test.go: Added test cases covering zero-padded formats (YYYY.0M0D.MICRO,
  YYYY.0M.MICRO) including cross-date scenarios (the exact user-reported bug scenario).

[Copilot]

Pull request overview

This PR improves Calendar Versioning (CalVer) behavior so zero-padded versions (e.g., 2026.0302.0) are preserved when rendering/tagging versions, and attempts to ensure version files are bumped appropriately in CalVer mode.

Changes:

• Update (*semv).Naked() to preserve the original CalVer string (including zero-padding) when a parsed calver.Calver instance isn’t available.
• In tagpr.Run, add a CalVer-specific path intended to bump configured version files after reloading config (post cherry-pick).
• Add tests covering cross-date CalVer bumping and zero-padding preservation for Naked()/Tag().

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tagpr.go	Adds CalVer-specific version-file bumping behavior after config reload in Run().
semver.go	Adjusts Naked() to preserve original CalVer formatting (zero-padding) when cv is nil.
calver_test.go	Adds regression tests for CalVer cross-date behavior and zero-padding preservation.

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