fix: respect config film_type to force negative inversion

NegativeDetector now accepts an optional forced FilmType. When
film_type != "auto" in config.ini, auto-detection is skipped and
the configured type is applied directly. build_pipeline() in
CliRunner maps c41→ColorNegative and bw→BWNegative accordingly.

Default config changed from film_type=auto to film_type=c41 to
match the project's primary use case (C-41 color negatives).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/cli/CliRunner.cpp

@@ -1,5 +1,6 @@
 #include "CliRunner.h"
 
+#include "../converter/pipeline/ImageData.h"
 #include "../converter/rawloader/RawLoader.h"
 #include "../converter/preprocess/Preprocessor.h"
 #include "../converter/negative/NegativeDetector.h"
@@ -269,8 +270,17 @@ std::vector<std::filesystem::path> CliRunner::collect_files(
 Pipeline CliRunner::build_pipeline(const AppConfig& app_cfg) {
     Pipeline pipeline;
 
+    // Resolve forced film type from config.
+    // "auto" → Unknown (auto-detection), "c41" → ColorNegative, "bw" → BWNegative.
+    FilmType forced_film = FilmType::Unknown;
+    if (app_cfg.conversion.film_type == "c41") {
+        forced_film = FilmType::ColorNegative;
+    } else if (app_cfg.conversion.film_type == "bw") {
+        forced_film = FilmType::BWNegative;
+    }
+
     pipeline.add_stage(std::make_unique<Preprocessor>());
-    pipeline.add_stage(std::make_unique<NegativeDetector>());
+    pipeline.add_stage(std::make_unique<NegativeDetector>(forced_film));
 
     if (app_cfg.conversion.invert) {
         pipeline.add_stage(std::make_unique<Inverter>());

src/converter/negative/NegativeDetector.cpp

@@ -13,6 +13,23 @@ StageResult NegativeDetector::process(ImageData data) const {
             ErrorCode::DetectionFailed, "NegativeDetector received empty image"));
     }
 
+    // If the caller forced a specific film type, skip analysis entirely.
+    if (forced_type_ != FilmType::Unknown) {
+        data.film_type = forced_type_;
+        std::cout << std::format("[Detect] Film type forced by config: {}",
+            [&] {
+                switch (data.film_type) {
+                    case FilmType::ColorNegative: return "Color Negative (C-41)";
+                    case FilmType::BWNegative:    return "B&W Negative";
+                    case FilmType::ColorPositive: return "Color Positive (Slide)";
+                    case FilmType::BWPositive:    return "B&W Positive";
+                    default:                      return "Unknown";
+                }
+            }()) << std::endl;
+        return data;
+    }
+
+    // Auto-detection: histogram + orange mask analysis.
     const bool is_negative = is_negative_histogram(data.rgb);
     const bool is_bw = is_monochrome(data.rgb);
 

src/converter/negative/NegativeDetector.h

@@ -13,7 +13,11 @@ namespace photoconv {
  * The detected FilmType is stored in ImageData::film_type for use
  * by the Invert and Color Correction stages.
  *
- * Detection strategy:
+ * When constructed with a forced FilmType (anything other than Unknown),
+ * the stage skips automatic analysis and sets that type directly.
+ * This allows the config's film_type setting to override auto-detection.
+ *
+ * Detection strategy (auto mode):
  * 1. Compute per-channel histograms
  * 2. Analyze distribution skewness (negatives have inverted distributions)
  * 3. Check for C-41 orange mask (dominant red/orange in unexposed regions)
@@ -27,13 +31,27 @@ public:
     /// Minimum saturation to distinguish color from B&W.
     static constexpr float kColorSaturationThreshold = 15.0f;
 
+    /**
+     * @brief Construct in auto-detection mode.
+     */
     NegativeDetector() = default;
+
+    /**
+     * @brief Construct with a forced film type (skips auto-detection).
+     *
+     * @param forced_type Film type to force. Use FilmType::Unknown for auto mode.
+     */
+    explicit NegativeDetector(FilmType forced_type) : forced_type_(forced_type) {}
+
     ~NegativeDetector() override = default;
 
     [[nodiscard]] StageResult process(ImageData data) const override;
     [[nodiscard]] std::string name() const override { return "Detect"; }
 
 private:
+    /// When set to anything other than Unknown, auto-detection is skipped.
+    FilmType forced_type_ = FilmType::Unknown;
+
     /**
      * @brief Analyze histogram to detect inverted (negative) distribution.
      *