fix(cli): restore direct script cache behavior

Author: GaspardKirira

include/vix/cli/commands/run/RunDetail.hpp

@@ -941,19 +941,22 @@ namespace vix::commands::RunCommand::detail
   // ===========================================================================
 
   /**
-   * @brief Return the file modification time as Unix nanoseconds.
+   * @brief Return a stable, order-preserving file modification timestamp.
    *
-   * std::filesystem::file_time_type may use a clock whose epoch differs from
-   * std::chrono::system_clock. Converting file_time_type::time_since_epoch()
-   * directly to an unsigned Unix timestamp is therefore not portable and can
-   * yield negative values for perfectly valid files.
+   * std::filesystem::file_time_type may use an implementation-defined epoch,
+   * so its timestamp must not be interpreted directly as a Unix timestamp.
    *
-   * This helper translates the filesystem clock to system_clock before
-   * producing a nanosecond timestamp.
+   * For cache fingerprints we need the value to be stable across repeated
+   * reads of the same file. For timestamp comparisons we also need the encoded
+   * unsigned value to preserve the ordering of the underlying signed
+   * filesystem-clock timestamp.
+   *
+   * Flipping the sign bit maps the signed nanosecond representation into an
+   * unsigned value while preserving chronological ordering.
    *
    * @param p File whose modification time should be queried.
    * @param ec Receives any filesystem error.
-   * @return Modification time in Unix nanoseconds, or 0 on failure.
+   * @return Stable encoded modification timestamp, or 0 on failure.
    */
   inline std::uint64_t file_mtime_ns(
       const fs::path &p,
@@ -971,26 +974,16 @@ namespace vix::commands::RunCommand::detail
 
     using namespace std::chrono;
 
-    const auto fileNow =
-        fs::file_time_type::clock::now();
-
-    const auto systemNow =
-        system_clock::now();
-
-    const auto systemTime =
-        systemNow +
-        duration_cast<system_clock::duration>(
-            fileTime - fileNow);
-
     const auto ns =
         duration_cast<nanoseconds>(
-            systemTime.time_since_epoch())
+            fileTime.time_since_epoch())
             .count();
 
-    if (ns <= 0)
-      return 0;
+    const auto signedNs =
+        static_cast<std::int64_t>(ns);
 
-    return static_cast<std::uint64_t>(ns);
+    return static_cast<std::uint64_t>(signedNs) ^
+           (std::uint64_t{1} << 63);
   }
 
   /**

src/commands/run/detail/DirectScriptRunner.cpp

@@ -1186,20 +1186,23 @@ namespace vix::commands::RunCommand::detail
     out.needsRebuild = true;
 
     /*
-     * A deterministic failed build is also a valid cache result.
+     * A previous compilation failure may keep its diagnostic payload,
+     * but it must never suppress a future compilation attempt.
      *
-     * Check it before requiring the binary: failed compilations and
-     * link steps intentionally have no executable.
+     * Source files, generated headers, compiler state, or external
+     * dependencies may have changed independently of the cached failure,
+     * so failed builds are informational only and are not cache hits.
      */
     if (load_direct_failure_cache(
             plan,
             out))
     {
-      out.cacheHit = true;
-      out.cachedFailure = true;
-      out.needsRebuild = false;
+      out.cacheHit = false;
+      out.cachedFailure = false;
+      out.needsRebuild = true;
+      out.cachedFailureExitCode = 0;
       out.rebuildReason =
-          "cached compile failure";
+          "previous compile failure";
 
       return out;
     }

src/commands/run/detail/ScriptProbe.cpp

@@ -919,23 +919,6 @@ namespace vix::commands::RunCommand::detail
       return out;
     }
 
-    /*
-     * Direct compilation does not yet have a reliable representation of
-     * Vix's transitive runtime link graph.
-     *
-     * Keep plain standalone C++ on the fast Direct path, but route scripts
-     * using Vix through the generated CMake project where exported Vix
-     * targets provide the complete link graph.
-     */
-    if (out.features.usesVix)
-    {
-      out.strategy = ScriptExecutionStrategy::CMakeFallback;
-      out.fallbackReason = ScriptFallbackReason::UsesVixRuntime;
-      out.canUseDirectCompile = false;
-      out.shouldUseCMakeFallback = true;
-      return out;
-    }
-
     const bool allowDirect =
         !unsupportedFlags &&
         (!out.usesCompiledDeps || onlyVixIncludesAdded) &&