Split packages: Breaks/Replaces the monolithic ecce below this version, computed each configure

ECCE_MONOLITHIC_BREAK_VERSION was a CACHE variable defaulting to the
version, so it froze at the build directory's first configure (8.17.0).
Installing the 8.18.0 split packages over an 8.17.x monolithic ecce then
stopped on file conflicts, and the metapackage's unpack removed the old
files. ECCE_MONOLITHIC_BREAK_VERSION_OVERRIDE still sets it explicitly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0167GfjZWhsGJh9at1F9sc9y
(cherry picked from commit ac76fbf)

Author: andy

CMakeLists.txt

@@ -1343,8 +1343,14 @@ if(ECCE_SPLIT_PACKAGES)
   # whichever release first ships split packages is the real boundary,
   # and that is a decision for whoever cuts that release, not something
   # to hardcode here now.
-  set(ECCE_MONOLITHIC_BREAK_VERSION "${_ecce_deb_version}" CACHE STRING
-      "ecce-client/ecce-server Break+Replace the monolithic ecce package for versions strictly below this (Debian: 'ecce (<< VERSION)'; RPM: 'ecce < VERSION'). Defaults to this build's own version, i.e. the first split release breaks/replaces every version older than itself.")
+  # Not a cache variable: a cached default froze at the version of the
+  # build directory's first configure (8.17.0), so an 8.17.x install was
+  # not replaced and the upgrade stopped on file conflicts.
+  if(NOT ECCE_MONOLITHIC_BREAK_VERSION_OVERRIDE)
+    set(ECCE_MONOLITHIC_BREAK_VERSION "${_ecce_deb_version}")
+  else()
+    set(ECCE_MONOLITHIC_BREAK_VERSION "${ECCE_MONOLITHIC_BREAK_VERSION_OVERRIDE}")
+  endif()
 
   # CPack falls back to the monolithic-build globals below (Depends,
   # Recommends, Suggests) for any component that doesn't set its own --