feat: multi-instance closures, richer stdlib, and Rust-style diagnostics

Introduce fat function pointers (BuxFn {code, env}) so capturing closures
are heap-allocated per value in both bootstrap and selfhost. Expand
Array/Map/Set/String/Test/Result APIs, add proper tuple codegen and
error snippets with multi-char underlines, golden diagnostic tests, and
LSP diagnostics via buxc check.

tests/error_golden/run.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+# Golden tests for Rust-style compiler diagnostics.
+# Usage: from repo root: tests/error_golden/run.sh [path/to/buxc]
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+BUXC="${1:-$ROOT/buxc}"
+DIR="$(cd "$(dirname "$0")" && pwd)"
+
+if [[ ! -x "$BUXC" ]]; then
+  echo "error: buxc not found at $BUXC (run make build first)"
+  exit 1
+fi
+
+passed=0
+failed=0
+
+normalize() {
+  # Replace absolute path prefix with FILE, drop trailing blank lines
+  sed -E \
+    -e "s|$DIR/[^:]+:|FILE:|g" \
+    -e "s|$ROOT/[^:]+:|FILE:|g" \
+    -e "s|//+|/|g" \
+    | sed -e :a -e '/^\n*$/{$d;N;ba' -e '}'
+}
+
+for case_dir in "$DIR"/*/; do
+  name="$(basename "$case_dir")"
+  [[ -f "$case_dir/expected.err" ]] || continue
+  [[ -f "$case_dir/bux.toml" ]] || continue
+
+  out="$("$BUXC" build "$case_dir" --color off 2>&1 || true)"
+  got="$(printf '%s\n' "$out" | normalize)"
+  exp="$(cat "$case_dir/expected.err")"
+
+  # Compare ignoring full absolute path differences already normalized
+  if [[ "$got" == "$exp" ]]; then
+    echo "  PASS $name"
+    passed=$((passed + 1))
+  else
+    echo "  FAIL $name"
+    echo "---- expected ----"
+    printf '%s\n' "$exp"
+    echo "---- got ----"
+    printf '%s\n' "$got"
+    echo "--------------"
+    failed=$((failed + 1))
+  fi
+done
+
+echo "Error golden tests: $passed passed, $failed failed"
+if [[ $failed -gt 0 ]]; then
+  exit 1
+fi

tests/error_golden/type_mismatch/bux.toml

@@ -0,0 +1,7 @@
+[Package]
+Name    = "type_mismatch"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"

tests/error_golden/type_mismatch/expected.err

@@ -0,0 +1,7 @@
+error: type errors in project
+error: cannot assign String to int
+  --> FILE:4:18
+    |
+ 4 |     let x: int = "hello";
+    |                  ^^^^^^^
+   = help: ensure the right-hand side type matches the left-hand side

tests/error_golden/type_mismatch/src/Main.bux

@@ -0,0 +1,6 @@
+import Std::Io::PrintLine;
+
+func Main() -> int {
+    let x: int = "hello";
+    return 0;
+}

tests/error_golden/undeclared/bux.toml

@@ -0,0 +1,7 @@
+[Package]
+Name    = "undeclared"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"

tests/error_golden/undeclared/expected.err

@@ -0,0 +1,7 @@
+error: type errors in project
+error: undeclared identifier 'noSuchVar'
+  --> FILE:4:15
+    |
+ 4 |     PrintLine(noSuchVar);
+    |               ^^^^^^^^^
+   = help: check the spelling, or import the symbol from the right module

tests/error_golden/undeclared/src/Main.bux

@@ -0,0 +1,6 @@
+import Std::Io::PrintLine;
+
+func Main() -> int {
+    PrintLine(noSuchVar);
+    return 0;
+}