Update existing golden tests to avoid unused assignment warnings

Add variable reads after assignments in test files that focus on other
checks (type errors, reassignment errors) so they don't incidentally
trigger the new unused assignment check.

https://claude.ai/code/session_01CwKU5QxyLPjEf5m7ybdmQJ

Author: claude

src/test_files/check/assign_add.gdn

@@ -3,9 +3,11 @@ public fun foo() {
     x += 1 // OK
 
     x += "" // bad
+    let _a = x
 
     let y = "abc"
     y -= 2 // bad
+    let _b = y
 }
 
 // args: check
@@ -16,26 +18,14 @@ public fun foo() {
 //  3|     x += 1 // OK
 //  4| 
 //  5|     x += "" // bad
-//  6|          ^^
-//  7|     let y = "abc"
+//   |          ^^
+//  6|     let _a = x
 // 
 // Error: `-=` can only be used with `Int` variables, but got `String`.
-// --| src/test_files/check/assign_add.gdn:8:5
-//  7|     let y = "abc"
-//  8|     y -= 2 // bad
-//  9| }   ^
-// 
-// Warning: `x` is not read after this assignment.
-// --| src/test_files/check/assign_add.gdn:5:5
-//  3|     x += 1 // OK
-//  4| 
-//  5|     x += "" // bad
-//  6|     ^^^^^^^
-//  7|     let y = "abc"
-// 
-// Warning: `y` is not read after this assignment.
-// --| src/test_files/check/assign_add.gdn:8:5
-//  7|     let y = "abc"
-//  8|     y -= 2 // bad
-//  9| }   ^^^^^^
+// --| src/test_files/check/assign_add.gdn:9:5
+//  8|     let y = "abc"
+//  9|     y -= 2 // bad
+//   |     ^
+// 10|     let _b = y
+// 11| }
 

src/test_files/check/assign_returns_unit.gdn

@@ -1,15 +1,7 @@
 public fun foo(): Unit {
     let i = 0
     i = 2
+    let _ = i
 }
 
 // args: check
-// expected exit status: 1
-// expected stdout:
-// Warning: `i` is not read after this assignment.
-// -| src/test_files/check/assign_returns_unit.gdn:3:5
-// 1| public fun foo(): Unit {
-// 2|     let i = 0
-// 3|     i = 2
-// 4| }   ^^^^^
-

src/test_files/check/assign_toplevel_fun_name.gdn

@@ -1,5 +1,6 @@
 public fun foo() {
   bar = 1
+  let _ = bar
 }
 
 public fun bar() {}
@@ -11,17 +12,15 @@ public fun bar() {}
 // -| src/test_files/check/assign_toplevel_fun_name.gdn:2:3
 // 1| public fun foo() {
 // 2|   bar = 1
-// 3| } ^^^
+//  |   ^^^
+// 3|   let _ = bar
+// 4| }
 // 
 // Error: Expected `Fun<(), Unit>`, but got `Int`.
 // -| src/test_files/check/assign_toplevel_fun_name.gdn:2:9
 // 1| public fun foo() {
 // 2|   bar = 1
-// 3| }       ^
-// 
-// Warning: `bar` is not read after this assignment.
-// -| src/test_files/check/assign_toplevel_fun_name.gdn:2:3
-// 1| public fun foo() {
-// 2|   bar = 1
-// 3| } ^^^^^^^
+//  |         ^
+// 3|   let _ = bar
+// 4| }
 

src/test_files/check/assign_update_error_type.gdn

@@ -1,39 +1,28 @@
 public fun foo() {
   let x = todo()
   x += 1
+  let _a = x
 
   let y = no_such_func()
   y += 1
+  let _b = y
 }
 
 // args: check
 // expected exit status: 1
 // expected stdout:
 // Warning: Unfinished code.
-// -| src/test_files/check/assign_update_error_type.gdn:2:11
-// 1| public fun foo() {
-// 2|   let x = todo()
-// 3|   x += 1  ^^^^^^
+// --| src/test_files/check/assign_update_error_type.gdn:2:11
+//  1| public fun foo() {
+//  2|   let x = todo()
+//  3|   x += 1  ^^^^^^
+//  4|   let _a = x
 // 
 // Error: Unbound symbol: `no_such_func`
-// -| src/test_files/check/assign_update_error_type.gdn:5:11
-// 3|   x += 1
-// 4| 
-// 5|   let y = no_such_func()
-// 6|   y += 1  ^^^^^^^^^^^^
-// 7| }
-// 
-// Warning: `x` is not read after this assignment.
-// -| src/test_files/check/assign_update_error_type.gdn:3:3
-// 1| public fun foo() {
-// 2|   let x = todo()
-// 3|   x += 1
-// 4|   ^^^^^^
-// 5|   let y = no_such_func()
-// 
-// Warning: `y` is not read after this assignment.
-// -| src/test_files/check/assign_update_error_type.gdn:6:3
-// 5|   let y = no_such_func()
-// 6|   y += 1
-// 7| } ^^^^^^
+// --| src/test_files/check/assign_update_error_type.gdn:6:11
+//  4|   let _a = x
+//  5| 
+//  6|   let y = no_such_func()
+//  7|   y += 1  ^^^^^^^^^^^^
+//  8|   let _b = y
 

src/test_files/check/assign_wrong_type.gdn

@@ -1,6 +1,7 @@
 public fun foo() {
     let x = 1
     x = ""
+    let _ = x
 }
 
 // args: check
@@ -11,12 +12,7 @@ public fun foo() {
 // 1| public fun foo() {
 // 2|     let x = 1
 // 3|     x = ""
-// 4| }       ^^
-// 
-// Warning: `x` is not read after this assignment.
-// -| src/test_files/check/assign_wrong_type.gdn:3:5
-// 1| public fun foo() {
-// 2|     let x = 1
-// 3|     x = ""
-// 4| }   ^^^^^^
+//  |         ^^
+// 4|     let _ = x
+// 5| }