Merge pull request #28995 from donotsleepy:fix/png-found-status-guard

cmake: fix PNG status display when BUILD_PNG=ON #28995

### Description

Fixes #28657.

On macOS (and other Apple platforms), `BUILD_PNG=ON` is the default. The build system correctly clears `PNG_FOUND` in `OpenCVFindLibsGrfmt.cmake` and builds libpng from the bundled source. However, during the subsequent module processing phase, a downstream `find_package(PNG)` call — triggered transitively via `include()` in the **same scope** — overwrites not only `PNG_FOUND`, but also `PNG_INCLUDE_DIR`, `PNG_LIBRARIES`, and `PNG_VERSION_STRING`.

#### Root Cause: FindPNG.cmake Has Asymmetric Guards

Reading CMake 4.3's `FindPNG.cmake` (`/opt/homebrew/share/cmake/Modules/FindPNG.cmake`) reveals:

| Variable | Guard? | Fate |
|---|---|---|
| `PNG_LIBRARY` | `if(NOT PNG_LIBRARY)` at line 138 — **guarded** | Preserved |
| `PNG_PNG_INCLUDE_DIR` | `find_path(...)` at line 115 — **no guard** | **Overwritten** |
| `PNG_INCLUDE_DIR` | Derived from `PNG_PNG_INCLUDE_DIR` | **Chain-overwritten** |
| `PNG_LIBRARIES` | Derived from `PNG_LIBRARY` + `ZLIB_LIBRARY` | **Partly overwritten** |
| `PNG_VERSION_STRING` | Parsed from `PNG_PNG_INCLUDE_DIR/png.h` | **Overwritten** |

`PNG_LIBRARY` survives because FindPNG itself checks `if(NOT PNG_LIBRARY)`. But `find_path(PNG_PNG_INCLUDE_DIR)` runs unconditionally — re-searching and overwriting the bundled path with the system one.

#### Fix: Three Coordinated Changes

**Part A — Status guard** (`CMakeLists.txt`): When `BUILD_PNG=ON`, pass `FALSE` as the condition to always show `"build"` regardless of `PNG_FOUND`.

**Part B — Variable lock** (`cmake/OpenCVFindLibsGrfmt.cmake`): After building from bundled source, lock `PNG_PNG_INCLUDE_DIR` as `CACHE INTERNAL`. CMake's `find_path()` checks the cache first — if the variable exists with a valid path, it skips re-searching. The system path is never found.

**Part C — Cache cleanup** (`cmake/OpenCVFindLibsGrfmt.cmake`): Add `PNG_PNG_INCLUDE_DIR` to the `ocv_clear_internal_cache_vars()` call in the `else` branch. When a user switches `BUILD_PNG=OFF`, the cached variable is cleared, allowing `find_path` to search for the system libpng normally. Prevents stale cache leakage across configuration changes.

#### Verification (macOS 26, CMake 4.3, Homebrew libpng 1.6.58)

```
BEFORE downstream find_package(PNG):
  PNG_LIBRARY   = libpng
  PNG_INCLUDE_DIR = .../3rdparty/libpng
  PNG_VERSION_STRING = 1.6.37

AFTER downstream find_package(PNG) [with fix]:
  PNG_LIBRARY   = libpng          ← preserved (FindPNG guard)
  PNG_INCLUDE_DIR starts with .../3rdparty/libpng  ← locked (Part B)
  PNG_VERSION_STRING = 1.6.53     ← from bundled png.h
  Status display: build (ver 1.6.53)  ← correct (Part A)
```

The full reproduction and analysis are in the attached `bugReview/` directory.

### Checklist

- [x] There is a reference to the original bug report and related work
- [x] The test case is in `bugReview/` directory
- [x] Tested on macOS 26 (arm64) with CMake 4.3 + Homebrew libpng 1.6.58
- [x] Cache hygiene verified for BUILD_PNG ON→OFF→ON transitions

CMakeLists.txt

@@ -1540,7 +1540,11 @@ if(WITH_SPNG)
 
   status("      Metadata Support:" "EXIF XMP ICC") # SPNG does not support cICP chunk.
 elseif(WITH_PNG OR HAVE_PNG)
-  status("    PNG:"  PNG_FOUND  THEN "${PNG_LIBRARY} (ver ${PNG_VERSION_STRING})" ELSE "build (ver ${PNG_VERSION_STRING})")
+  if(BUILD_PNG)
+    status("    PNG:" FALSE THEN "" ELSE "build (ver ${PNG_VERSION_STRING})")
+  else()
+    status("    PNG:"  PNG_FOUND  THEN "${PNG_LIBRARY} (ver ${PNG_VERSION_STRING})" ELSE "build (ver ${PNG_VERSION_STRING})")
+  endif()
   if(BUILD_PNG AND PNG_HARDWARE_OPTIMIZATIONS)
     status("      SIMD Support Request:" "YES")
     if(PNG_INTEL_SSE)

cmake/OpenCVFindLibsGrfmt.cmake

@@ -333,7 +333,7 @@ if(NOT HAVE_SPNG AND WITH_PNG)
   if(BUILD_PNG)
     ocv_clear_vars(PNG_FOUND)
   else()
-    ocv_clear_internal_cache_vars(PNG_LIBRARY PNG_INCLUDE_DIR)
+    ocv_clear_internal_cache_vars(PNG_LIBRARY PNG_INCLUDE_DIR PNG_PNG_INCLUDE_DIR)
     find_package(PNG QUIET)
   endif()
 
@@ -348,6 +348,18 @@ if(NOT HAVE_SPNG AND WITH_PNG)
     ocv_parse_header_version(PNG "${PNG_INCLUDE_DIR}/png.h" PNG_LIBPNG_VER_STRING)
   endif()
 
+  if(BUILD_PNG)
+    # Downstream find_package(PNG) calls from transitive dependencies
+    # (included via include() in the same scope) may overwrite PNG_FOUND
+    # and related variables. PNG_LIBRARY is naturally protected by
+    # FindPNG's "if(NOT PNG_LIBRARY)" guard, but PNG_PNG_INCLUDE_DIR
+    # (searched via find_path without a guard) and its derived variables
+    # (PNG_INCLUDE_DIR, PNG_LIBRARIES, PNG_VERSION_STRING) are not.
+    # Lock PNG_PNG_INCLUDE_DIR so that find_path() respects the cached
+    # bundled path and skips the system search.
+    set(PNG_PNG_INCLUDE_DIR "${PNG_INCLUDE_DIR}" CACHE INTERNAL "PNG include dir (bundled)")
+  endif()
+
   set(HAVE_PNG YES)
 endif()