Add a version epoch to supersede Raspberry Pi's libcamera

[DaniusKalv]

Summary

• We pin our patches to a fixed upstream snapshot, so Raspberry Pi's
  monotonic +rptYYYYMMDD date eventually overtakes ours. Without an
  epoch, apt keeps RPi's newer libcamera as an already-satisfied
  dependency of rpicam-apps and our camera support is silently dropped.
• Add a Debian epoch (1:) so Kurokesu's libcamera is unconditionally
  the higher version regardless of date
  (1:0.7.1+rpt20260429+krks1-4).
• The epoch is comparison-only: colons are illegal in git refs and
  rejected by upload-artifact, so the release tooling keeps it out of
  tags and asset filenames and applies it only to the .deb version
  apt compares:
  • _build.yml: strip the epoch for the orig tarball, source dir and
    asset name; the .deb keeps it via the packaging version.
  • release.yml / release.sh: derive epoch-free tags and strip the
    epoch before the tag-vs-changelog check.
  • d/source/README.source: document the epoch in the version scheme
    and the tag convention.

Note

The epoch is permanent, removing it later is itself a downgrade. This
is intentional: Kurokesu's libcamera should always supersede stock.

Test plan

• ./release.sh (dry run) reports colon-free tags:
  v0.7.1+rpt20260429+krks1 and debian/0.7.1+rpt20260429+krks1-4.
• dpkg --compare-versions 1:0.7.1+rpt20260429+krks1-4 gt 0.7.1+rpt20260609-1
  holds (ours outranks RPi's trixie stock).
• Downstream gate (after this releases as -4): rebuild rpicam-apps
  against the pin and confirm a clean-Pi apt install rpicam-apps
  pulls Kurokesu's libcamera.