fix(build): declare ftxui under its canonical namespace

[Sunrisepeak]

Problem

mcpp.toml declares bare ftxui = "6.1.9", while the current index identity is compat.ftxui. The exact-selector contract in mcpp-community/mcpp#400 correctly maps a bare name only to the default mcpplibs namespace, so xlings self-builds fail instead of guessing another namespace.

The existing lockfile also records the stale mcpplibs.ftxui identity.

Acceptance

• declare ftxui as compat.ftxui in mcpp.toml;
• refresh the lock identity without unrelated dependency drift;
• prove xlings builds with the exact-selector mcpp from feat: converge template identity, runtime contracts, and release state mcpp-community/mcpp#400.

Related: mcpp-community/mcpp#398

[Sunrisepeak]

The exact-selector validation also found the same legacy spelling in the test-only dependency: bare [dev-dependencies] gtest = "1.15.2" resolves as (mcpplibs, gtest), while the published package is (compat, gtest). The companion PR will move both direct dependencies to their exact compat tables and validate build + tests against the exact-selector mcpp branch.