fix(xpkg): survive the full mcpp-index corpus — 42/42 descriptors parse strict

Dry-running the new lint over every existing mcpp-index descriptor found
three gaps, all fixed:

- list_xpkg_versions only accepted double-quoted version keys; tensorvia's
  ['0.1.1'] spelling made its versions invisible. Accept both quotes.
- linux/macosx/windows per-platform sub-tables are KNOWN schema keys (the
  current platform's body is merged before the key loop); they no longer
  read as unknown on other platforms.
- Form A descriptors (no mcpp segment; build info from the source's own
  mcpp.toml) are a valid form: `xpkg parse` reports form A and passes
  instead of failing on the missing segment.

Corpus check: all 42 pkgs/*.lua in mcpp-index now pass `xpkg parse`
(strict) with zero warnings.

Author: Sunrisepeak

src/cli/cmd_xpkg.cppm

@@ -95,6 +95,22 @@ export int cmd_xpkg_parse(const mcpplibs::cmdline::ParsedArgs& parsed) {
         return 1;
     }
 
+    // Form A descriptors carry no `mcpp = {}` table — build info comes
+    // from the fetched source's own mcpp.toml. Nothing further to parse.
+    auto field = mcpp::manifest::extract_mcpp_field(lua);
+    if (field.kind != mcpp::manifest::McppField::TableBody) {
+        if (asJson) {
+            std::println("{{\"namespace\":\"{}\",\"name\":\"{}\",\"form\":\"A\"}}",
+                         json_escape(id.ns), json_escape(id.name));
+        } else {
+            std::println("package    {} (namespace '{}')", fqn, id.ns);
+            std::println("form       A — no mcpp segment (build info from the "
+                         "source's mcpp.toml)");
+            std::println("parse OK");
+        }
+        return 0;
+    }
+
     // The parse users get at build time — same function, same grammar.
     auto m = mcpp::manifest::synthesize_from_xpkg_lua(lua, fqn, anyVersion);
     if (!m) {

src/manifest/xpkg.cppm

@@ -702,11 +702,12 @@ list_xpkg_versions(std::string_view luaContent, std::string_view platform) {
         if (text[q] == '[') {
             std::size_t r = q + 1;
             while (r < plat_end && (text[r] == ' ' || text[r] == '\t')) ++r;
-            if (r < plat_end && text[r] == '"') {
+            if (r < plat_end && (text[r] == '"' || text[r] == '\'')) {
+                const char quote = text[r];
                 ++r;
                 std::size_t key_start = r;
-                while (r < plat_end && text[r] != '"' && text[r] != '\n') ++r;
-                if (r < plat_end && text[r] == '"') {
+                while (r < plat_end && text[r] != quote && text[r] != '\n') ++r;
+                if (r < plat_end && text[r] == quote) {
                     versions.emplace_back(luaContent.substr(key_start, r - key_start));
                 }
             }
@@ -1182,6 +1183,12 @@ synthesize_from_xpkg_lua(std::string_view luaContent,
             }
             cur.consume('}');
         }
+        else if (key == "linux" || key == "macosx" || key == "windows") {
+            // Per-platform sub-table. The CURRENT platform's body was already
+            // appended to the segment before this loop (see above), so every
+            // spelling here is a known key: skip the table itself.
+            if (cur.peek() == '{') cur.skip_table();
+        }
         else if (key == "schema") {
             // Descriptor schema tag (e.g. "0.1") — accepted, currently
             // informational only.