Modernize CMake package config and install layout

[caitlinross]

Makes find_package(ROSS) a self-contained modern CMake package. Out-of-tree consumers can write:

find_package(ROSS REQUIRED)
target_link_libraries(myapp PRIVATE ROSS::ROSS)

and get include dirs, the MPI dependency (re-resolved on the consumer's machine, not baked in), and link libraries propagated transparently. Master's hand-written 3-line ROSSConfig.cmake had no version file, no find_dependency(MPI), no namespaced target, and lived at a non-canonical location requiring -DROSS_DIR hints to find.

What changed:

• New canonical config at <prefix>/lib/cmake/ROSS/ with proper version file, MPI dependency re-resolution, and ROSS::ROSS namespaced target. CMake discovers it from CMAKE_PREFIX_PATH automatically — no hints needed.
• Installed headers moved from flat <prefix>/include/ to <prefix>/include/ross/ to stop polluting the consumer include namespace with common names (config.h, buddy.h, lz4.h, io.h).
• Core target hygiene: dropped the nested PROJECT(ROSS C) in core/, attached public include dirs via target_include_directories(... BUILD_INTERFACE / INSTALL_INTERFACE), moved CMake helper modules to top-level cmake/.
• All install destinations honor GNUInstallDirs: the library, CMake
  config files, pkg-config file, headers, phold binaries, and the
  install-RPATH all derive from ${CMAKE_INSTALL_LIBDIR} /
  ${CMAKE_INSTALL_INCLUDEDIR} / ${CMAKE_INSTALL_BINDIR}. No effect on
  macOS or standard Linux layouts, but installs cleanly on Debian
  multiarch (lib/x86_64-linux-gnu) and RHEL/Fedora multilib (lib64).
• models/phold updated to install all 6 phold variants (master only installed phold) and dropped a dead BGPM branch.

Back-compat:

• ross.pc keeps working for pkg-config consumers. CODES rebuilds cleanly against the new install with no source changes .
• Library filename libROSS.{a,so} unchanged, cached pkgcfg_lib_ROSS_ROSS entries in consumer build dirs stay valid.
• Legacy find_package(ROSS) + -DROSS_DIR=<prefix>/lib callers keep working via a tiny shim at the old location; emits a deprecation warning and delegates to the canonical config (preserves the ROSS_INCLUDE_DIRS variable master set).
• Legacy target_link_libraries(... ROSS) callers keep working via a back-compat ALIAS in the new ROSSConfig.cmake.

---

Checklist

• Builds cleanly with -Wall and -Wextra
• CI is green
• Added a changelog fragment under Documentation/dev/, unless the change is invisible to anyone outside the PR (test refactors, internal renames, comment-only tweaks)
• Confirmed nothing in CODES breaks. CI has a minimal CODES build that should be green before merging.
• For new features: blog post on the ROSS website, with link in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.86%. Comparing base (427ed81) to head (961dd65).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #204   +/-   ##
=======================================
  Coverage   47.86%   47.86%           
=======================================
  Files          37       37           
  Lines        4745     4745           
  Branches      857      857           
=======================================
  Hits         2271     2271           
  Misses       2469     2469           
  Partials        5        5           

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