Enable need-check-nil and start the burn down

`Roles.new_role` returned the result of `error`, which made it nullable,
and `set_permission_group` returned nil for an unknown group, which
broke the config chain with an index error rather than saying so.

Positions and bounding boxes read back from the game always use the
named members, so they are narrowed once where they enter a function.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.emmyrc.json

@@ -18,7 +18,6 @@
     },
     "$comment-disable": "Carried over from the luals config, which had these as editor only rather than checked in ci",
     "$comment-unnecessary-if": "Disabled by the config fmtk generates for itself, the api union types make it unreliable",
-    "$comment-need-check-nil": "To be enabled once the backlog is cleared, a third of it is MapPosition and BoundingBox being aliased as struct|[double, double]",
     "diagnostics": {
         "globals": [ "__DebugAdapter", "__Profiler" ],
         "disable": [
@@ -27,7 +26,6 @@
             "invert-if",
             "missing-parameter",
             "missing-return-value",
-            "need-check-nil",
             "param-type-mismatch",
             "preferred-local-alias",
             "redefined-local",