feat(lang): a name is defined once, a local once per block, and a result is always returned

L1. Two vars, consts, enums, properties or events of one name in a program are an error naming
both places (functions already were); the first used to win silently. A let / var of a name its
own block already declared is an error (it used to shadow). A function with a result type whose
body can reach its end without a return is an error, asked structurally of the body - a return,
an if/else or a match with a default arm whose every branch ends - rather than handing back
whatever the result slot held. The game, the lab and every package example pass all three;
the lab had one harmless shadow, and the game's split screens had two real bugs of the kind.
examples/rejected/ holds the four refusals, checked by the test runner's new reject_case.
Seed regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/rejected/redeclared_local.ludic

@@ -0,0 +1,9 @@
+# redeclared_local.ludic - one block declaring a name twice. The second used to shadow the first
+# silently: a settings page compared its vegetation setting against the volumetric one for it.
+program RedeclaredLocal {
+  entry {
+    let v0 = 1
+    let v0 = 2
+    print(v0)
+  }
+}