Report the resolved compression-fusion decision in CLI JSON

FZGM CLI JSON reports now include the resolved fusion policy,
legal-domain count, installed implementation and stage coverage, and an
explicit fallback reason, so corpus results can prove whether Auto
actually specialized a pipeline.

Author: skyler-ruiter

src/utils/cli/cli.cpp

@@ -782,6 +782,15 @@ static std::string strategy_str(MemoryStrategy s) {
     return s == MemoryStrategy::MINIMAL ? "MINIMAL" : "PREALLOCATE";
 }
 
+static const char* fusion_policy_str(FusionPolicy p) {
+    switch (p) {
+        case FusionPolicy::Off:   return "off";
+        case FusionPolicy::Auto:  return "auto";
+        case FusionPolicy::Force: return "force";
+    }
+    return "off";
+}
+
 static const char* operation_str(CliOperation op) {
     switch (op) {
         case CliOperation::Compress:   return "compress";
@@ -827,6 +836,16 @@ static void fill_from_pipeline(fz::cli::ReportData& d, const Pipeline& p) {
     // key order would make otherwise-identical reports diff.
     std::sort(d.run_notes.begin(), d.run_notes.end(),
               [](const auto& a2, const auto& b2) { return a2.first < b2.first; });
+
+    const FusionInfo& fusion = p.getFusionInfo();
+    d.has_fusion = true;
+    d.fusion_policy = fusion_policy_str(fusion.policy);
+    d.fusion_legal_group_count = fusion.legal_group_count;
+    d.fusion_fallback_reason = fusion.fallback_reason;
+    d.fusion_installed_groups.clear();
+    for (const auto& group : fusion.installed_groups) {
+        d.fusion_installed_groups.push_back({group.implementation, group.stages});
+    }
 }
 
 // Append per-stage device timings from a perf result into the report.
@@ -1366,4 +1385,4 @@ int fzgmod_cli_main(int argc, char** argv) {
         print_root_usage(argc > 0 ? argv[0] : "fzgmod-cli");
         return 1;
     }
-}
+}

src/utils/cli/report_json.cpp

@@ -201,7 +201,7 @@ void write_report_json(const std::string& path, const ReportData& d) {
           << "\", \"device_ms\": " << num(s.device_ms) << " }";
     }
     o << (d.stages.empty() ? "]" : "\n  ]");
-    if (d.graph_requested || !d.stage_versions.empty() || !d.run_notes.empty()) o << ",";
+    if (d.graph_requested || !d.stage_versions.empty() || !d.run_notes.empty() || d.has_fusion) o << ",";
     o << "\n";
 
     // stage_versions: source fingerprint of each stage that ran.  Omitted entirely
@@ -216,7 +216,7 @@ void write_report_json(const std::string& path, const ReportData& d) {
             first = false;
         }
         o << "\n  }";
-        if (d.graph_requested || !d.run_notes.empty()) o << ",";
+        if (d.graph_requested || !d.run_notes.empty() || d.has_fusion) o << ",";
         o << "\n";
     }
 
@@ -236,6 +236,32 @@ void write_report_json(const std::string& path, const ReportData& d) {
             first = false;
         }
         o << "\n  }";
+        if (d.graph_requested || d.has_fusion) o << ",";
+        o << "\n";
+    }
+
+    if (d.has_fusion) {
+        size_t installed_stage_count = 0;
+        for (const auto& group : d.fusion_installed_groups)
+            installed_stage_count += group.stages.size();
+        o << "  \"fusion\": {\n";
+        o << "    \"policy\": \"" << esc(d.fusion_policy) << "\",\n";
+        o << "    \"legal_group_count\": " << d.fusion_legal_group_count << ",\n";
+        o << "    \"installed_group_count\": " << d.fusion_installed_groups.size() << ",\n";
+        o << "    \"installed_stage_count\": " << installed_stage_count << ",\n";
+        o << "    \"fallback_reason\": " << str_or_null(d.fusion_fallback_reason) << ",\n";
+        o << "    \"groups\": [";
+        for (size_t i = 0; i < d.fusion_installed_groups.size(); ++i) {
+            const auto& group = d.fusion_installed_groups[i];
+            o << (i ? "," : "") << "\n      { \"implementation\": \""
+              << esc(group.implementation) << "\", \"stages\": [";
+            for (size_t j = 0; j < group.stages.size(); ++j) {
+                o << (j ? ", " : "") << "\"" << esc(group.stages[j]) << "\"";
+            }
+            o << "] }";
+        }
+        o << (d.fusion_installed_groups.empty() ? "]\n" : "\n    ]\n");
+        o << "  }";
         if (d.graph_requested) o << ",";
         o << "\n";
     }

src/utils/cli/report_json.h

@@ -44,6 +44,11 @@ struct PhaseTimingJson {
     std::vector<double> host_wall_ms;   ///< per-rep host wall time (ms)
 };
 
+struct FusionGroupJson {
+    std::string implementation;
+    std::vector<std::string> stages;
+};
+
 /// Everything the JSON needs, in plain types decoupled from CLI internals.
 struct ReportData {
     // ── tool ──
@@ -101,6 +106,13 @@ struct ReportData {
     /// case costs no bytes and an absent key reads as "no notes".
     std::vector<std::pair<std::string, std::vector<std::string>>> run_notes;
 
+    // ── finalize-time compression fusion decision ──
+    bool                         has_fusion = false;
+    std::string                  fusion_policy;
+    size_t                       fusion_legal_group_count = 0;
+    std::vector<FusionGroupJson> fusion_installed_groups;
+    std::string                  fusion_fallback_reason;
+
     // ── graph mode (benchmark only; omitted entirely when graph_requested is false) ──
     bool        graph_requested = false;
     bool        graph_active    = false;