docs(spec v3): clean templates entry + add cmdline workaround

Two changes per docs/20-package-descriptor-spec.md:

1. pkgs/m/mcpplibs.templates.lua — Form A cleanup:
   The upstream source (mcpp-community/templates) has its own
   mcpp.toml, so the per-package mcpp = {} segment is redundant.
   Removed it; descriptor is now pure xlings standard.

2. pkgs/c/mcpplibs.cmdline.lua — Form B workaround (NEW):
   Indexes the existing https://github.com/mcpplibs/cmdline @ 0.0.1
   which doesn't have mcpp.toml. The hand-written `mcpp = {}` block
   provides the missing build info. Drop the segment when upstream
   adds its own mcpp.toml.

Both descriptors are validated by the existing CI lint (no install
hooks, schema fields removed).

Author: Sunrisepeak

pkgs/c/mcpplibs.cmdline.lua

@@ -0,0 +1,43 @@
+-- Hand-written workaround entry.
+-- Upstream repo (mcpplibs/cmdline) has no mcpp.toml; the `mcpp = {}` block
+-- below carries the build info mcpp would otherwise read from mcpp.toml.
+-- Drop the `mcpp = {}` segment when the upstream repo adopts mcpp.toml natively.
+package = {
+    spec        = "1",
+    name        = "mcpplibs.cmdline",
+    description = "A simple command-line parsing library/framework for modern C++",
+    licenses    = {"Apache-2.0"},
+    repo        = "https://github.com/mcpplibs/cmdline",
+    type        = "package",
+
+    xpm = {
+        linux = {
+            ["0.0.1"] = {
+                url    = "https://github.com/mcpplibs/cmdline/archive/refs/tags/0.0.1.tar.gz",
+                sha256 = "3fb2f5495c1a144485b3cbb2e43e27059151633460f702af0f3851cbff387ef0",
+            },
+        },
+        macosx = {
+            ["0.0.1"] = {
+                url    = "https://github.com/mcpplibs/cmdline/archive/refs/tags/0.0.1.tar.gz",
+                sha256 = "3fb2f5495c1a144485b3cbb2e43e27059151633460f702af0f3851cbff387ef0",
+            },
+        },
+        windows = {
+            ["0.0.1"] = {
+                url    = "https://github.com/mcpplibs/cmdline/archive/refs/tags/0.0.1.tar.gz",
+                sha256 = "3fb2f5495c1a144485b3cbb2e43e27059151633460f702af0f3851cbff387ef0",
+            },
+        },
+    },
+
+    -- Workaround: upstream repo has no mcpp.toml. Drop this segment when it does.
+    mcpp = {
+        language   = "c++23",
+        import_std = true,
+        modules    = { "mcpplibs.cmdline" },
+        sources    = { "src/**/*.cppm" },
+        targets    = { ["cmdline"] = { kind = "lib" } },
+        deps       = { },
+    },
+}