fix(lint): report an unavailable AST once per run, not per (file, rule)

Linting a march whose PCMs are not built produced 102 findings — 34 files times
three AST rules — for a single fact: the project has not been built for that
configuration. The one actionable sentence was buried.

Now recorded per file and reported once, naming the rules that could not run,
how many files were affected, one example reason, and what to do about it. Still
one error, so the run still fails; --no-ast remains the way to proceed without
building.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

implementations/Crafter.Build-Lint.cpp

@@ -953,6 +953,9 @@ format` applies it to disk, and `crafter-build lint` reports where it would.
         return PathInsideRoot(p, cwd) ? p.lexically_relative(cwd) : p;
     };
 
+    std::map<fs::path, std::string> astFailures;
+    std::set<std::string_view> skippedAstRules;
+
     // Any rule reading Decls() needs this configuration's module PCMs, and a
     // parse without them is fatal rather than degraded. Produce them up front
     // rather than letting each file fail on its own, and cache the assembled
@@ -1020,8 +1023,12 @@ format` applies it to disk, and `crafter-build lint` reports where it would.
                 // from reporting it clean.
                 if (ctx.compileCommand.empty()) continue;
                 if (!ctx.AstAvailable()) {
-                    ctx.Report(0, std::format("rule '{}' needs an AST, which is unavailable: {}", rule->name, ctx.AstUnavailableReason()));
-                    ++summary.errors;
+                    // Recorded once per file and reported as a single grouped
+                    // error after the run. One missing PCM would otherwise
+                    // produce a finding per (file, rule) and bury the one fact
+                    // that matters — which is that a build has to happen first.
+                    astFailures.emplace(file, ctx.AstUnavailableReason());
+                    skippedAstRules.insert(rule->name);
                     continue;
                 }
             }
@@ -1113,6 +1120,18 @@ format` applies it to disk, and `crafter-build lint` reports where it would.
         }
     }
 
+    if (!astFailures.empty()) {
+        std::string rules;
+        for (std::string_view name : skippedAstRules) {
+            if (!rules.empty()) rules += ", ";
+            rules += name;
+        }
+        std::println(std::cerr, "lint: {} rule(s) needing an AST ({}) could not run on {} of {} file(s).", skippedAstRules.size(), rules, astFailures.size(), summary.filesLinted);
+        std::println(std::cerr, "      {}: {}", shown(astFailures.begin()->first).string(), astFailures.begin()->second);
+        std::println(std::cerr, "      Build the project first so the module PCMs exist, or pass --no-ast to skip these rules.");
+        ++summary.errors;
+    }
+
     std::sort(summary.findings.begin(), summary.findings.end(),
               [](const LintFinding& a, const LintFinding& b) {
                   return std::tie(a.file, a.line) < std::tie(b.file, b.line);