feat(selection-test): migrate to test-lib (manifest.lib + run_tests) (Test-Opt Phase 3.4)

Per ADR-0037 Tests-as-Libs Convention. Renamed on Gitea + moved
locally. History preserved.

init.lua

@@ -4,7 +4,9 @@
 local selection = require("lib-core.selection")
 local T         = engine.test
 
-function init(ctx)
+local M = {}
+
+function M.run_tests(ctx)
     -- Register / Unregister (3)
     local h1 = selection.register(function() return {x=0, y=0, w=10, h=10} end)
     T.assert(h1 ~= nil and h1 >= 1, "register returns positive handle")
@@ -66,3 +68,5 @@ function init(ctx)
     local ok = pcall(function() selection.set_drag_threshold(-1) end)
     T.assert(not ok, "set_drag_threshold(-1) errors")
 end
+
+return M