chore: bump version to 2026.10.3.1 (one past the engine-binding release at 2026.10.2.1)

The release at 2026.10.2.1 carries the build record's engine field and the
package-rooted dep-source classification. Bumping to 2026.10.3.1 so the next
publish-ecosystem run writes a fresh tag and the index PR is the single
canonical bump record.

Not touched:
- .xlings.json (still 2026.9.24.1): bumped AFTER release/ecosystem publish.
- ci-fresh-install.yml MCPP_PIN: derived from wait-index, never hand-edited.
- src/xlings/xlings.cppm::kXlingsVersion: 2026.9.30.1 (unchanged this iteration).
- check_version_pins.sh: OK; building 2026.10.3.1, bootstrapping 2026.9.24.1.

Author: speak-agent

mcpp.toml

@@ -1,6 +1,6 @@
 [package]
 name        = "mcpp"
-version = "2026.10.2.1"
+version = "2026.10.3.1"
 description = "Modern C++ build & package management tool"
 license     = "Apache-2.0"
 authors     = ["mcpp-community"]

modules/versioning/src/version.cppm

@@ -31,6 +31,6 @@ import std;
 
 export namespace mcpp {
 
-inline constexpr std::string_view MCPP_VERSION = "2026.10.2.1";
+inline constexpr std::string_view MCPP_VERSION = "2026.10.3.1";
 
 } // namespace mcpp