Number versions MAJOR.MINOR.PATCH in the version tool

Progmasoft repositories use Semantic Versioning since 2026-11-12, but
//tools:version still knew the four-number counters: given 0.4.0 it
wrote 0.4.0.0 everywhere, and --next stepped the fourth number.

- A Version is three numbers; ParseVersion takes exactly three, without
  leading zeros, and FormatVersion writes three.
- --next is the next patch release. It also follows a four-number
  version as the rule says: the fourth number goes and the patch rises,
  so 0.3.0.2 is followed by 0.3.1.
- ReleaseNotesOf finds a section by the version it was published as.
  Its fallback for "D = 0" read the fourth element of what is now a
  three-element array; it is gone.
- Tests for the new parsing, NextPatch and release-note lookup;
  docs/contributing.md and the tool's usage say what the tool does.

Claude-Session: https://claude.ai/code/session_01Ji7xZMYjc4zankoc7hGBE1

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