Pure refactor, no behavior change.
cmake/NSIS.template.in is the CPack template, and CPack substitutes the @...@
placeholders in that file only, so everything without a substitution moves into
cmake/nsis/ and is included from it:
installer-log.nsh the CockLog macro and the two writers behind it
process-guard.nsh running instance handling, installer and uninstaller copy
user-data.nsh backup/restore around the previous version's uninstaller
runtime-purge.nsh stale runtime purge, diagnostics and the holder probe
The page and section order, the Vars the included files share, .onInit (it names
the package version in the log), AbortInstall (its Sentry offer is substituted),
componentsPagePre and the registry bookkeeping stay in the template.
The includes resolve through NSIS_SOURCE_PATH, which the template already uses
for the bitmaps, and an include does not change the compile directory, so the
holder probe keeps its relative File path.
