early warn TCL support deprecate and Tcl 9.x warning (parflow#716)

## Summary

Marks TCL support as deprecated and adds early version-gating for the
Tcl 9.x incompatibility for generating PFIDB, replacing silent runtime
failures with configure-time warnings.

## Changes

- Issue a deprecation `WARNING` when TCL is found, directing users to
Python-based
  pftools and tracking issue parflow#707
- Raise a `WARNING` when Tcl ≥ 9.0 is detected as generating PFIDB is
unsupported on that version

## Motivation

Tcl 9.x silently breaks at runtime and does not generate PFIDB, with no
configure-time indication, leading to hard-to-diagnose failures. This
change surfaces the incompatibility immediately at configure time and
consistently nudges remaining TCL users toward the Python migration path
ahead of eventual removal.

## Testing

- [x] **Configured with Tcl 8.6:** deprecation warning issued, build
proceeds normally
- [x] **Configured with Tcl 9.0:** both warnings issued, build proceeds
with known limitations noted

## Related issues

-  parflow#641
-  parflow#707

Author: mfahdaz

CMakeLists.txt

@@ -281,14 +281,41 @@ if ( ${PARFLOW_AMPS_LAYER} IN_LIST PARFLOW_AMPS_LAYER_REQUIRE_MPI )
 endif ( ${PARFLOW_AMPS_LAYER} IN_LIST PARFLOW_AMPS_LAYER_REQUIRE_MPI )
 
 find_package(TCL QUIET)
-if (${TCL_FOUND})
-  set(PARFLOW_HAVE_TCL "yes")
-  set(HAVE_TCL ${PARFLOW_HAVE_TCL})
-else (${TCL_FOUND})
-  if (${PARFLOW_ENABLE_TOOLS})
+if(TCL_FOUND)
+  # Warn: TCL support is deprecated and will be removed in a future release.
+  # Tracking issue: https://github.com/parflow/parflow/issues/707<N>
+  message(WARNING
+    "TCL support in ParFlow is deprecated and will be removed in a future release. "
+    "Users are encouraged to migrate to the Python-based pftools. "
+    "See https://github.com/parflow/parflow/issues/707 for migration guidance."
+  )
+  set(PARFLOW_HAVE_TCL TRUE)
+  set(HAVE_TCL "${PARFLOW_HAVE_TCL}")
+
+  # Determine TCL version
+  # TCL_VERSION is the canonical macro in tcl.h across all supported versions.
+  if(TCL_INCLUDE_PATH AND EXISTS "${TCL_INCLUDE_PATH}/tcl.h")
+    file(STRINGS "${TCL_INCLUDE_PATH}/tcl.h" _tcl_version_line
+         REGEX "^#[ \t]*define[ \t]+TCL_VERSION[ \t]+\"[^\"]+\"")
+    if(_tcl_version_line)
+      string(REGEX REPLACE ".*\"([^\"]+)\".*" "\\1" PARFLOW_TCL_VERSION "${_tcl_version_line}")
+    unset(_tcl_version_line)
+    endif(_tcl_version_line)
+  endif(TCL_INCLUDE_PATH AND EXISTS "${TCL_INCLUDE_PATH}/tcl.h")
+  # Tcl 9.x removed PFIDB support. Warn early rather than fail at runtime
+  if(PARFLOW_TCL_VERSION VERSION_GREATER_EQUAL "9.0")
+    message(WARNING
+      "TCL ${PARFLOW_TCL_VERSION} detected (>= 9.0). "
+      "ParFlow will compile, but creating or running PFIDB files is not supported with Tcl 9.x. "
+      "Migrate to Python-based pftools or use Tcl 8.x."
+    )
+  endif()
+
+else(TCL_FOUND)
+  if(PARFLOW_ENABLE_TOOLS)
     message(FATAL_ERROR "TCL is required for building pftools")
-  endif (${PARFLOW_ENABLE_TOOLS})
-endif (${TCL_FOUND})
+  endif()
+endif(TCL_FOUND)
 
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 # SILO