`patch-add` overlay appends `PatchN:` tag at end of preamble instead of grouping with existing Patch tags

[liunan-ms]

Summary
When the patch-add overlay adds a patch to a spec that has no %patchlist section, the new PatchN: tag is appended at the very end of the preamble (just before %description) rather than being placed alongside the existing Patch/PatchN tags. This produces valid but awkwardly-placed output.

Example
For libsolv.spec, which has an upstream Patch: tag near the top of the preamble, running the patch-add overlay:

[[components.libsolv.overlays]]
description = "Make file time available for dnf plugins, a prerequisite for virtual snapshots"
type = "patch-add"
source = "0002-Make-file-time-available-for-dnf-plugins.patch"

inserts Patch1: at ~L92 (after the %if %{with zchunk}...%endif BuildRequires block, immediately before %description), instead of next to the existing Patch: tag at ~L70.

Root cause

• The patch-add overlay calls Spec.AddPatchEntry (edit.go:653).
• When there is no %patchlist section, it falls back to AddTag(...) (edit.go:172).
• AddTag calls AppendLinesToSection, which always inserts at the end of the section (edit.go:473).

Acceptance criteria

• New patches added via patch-add are placed next to existing Patch/PatchN tags. (cosmetic change)
• Specs with a %patchlist section continue to append there.
• Specs with no existing patch tags behave as today.