check: over-strict-mode lint (iter 19a)

When a fn-param is annotated (own T) but the body never consumes
it (consume_count == 0) and never destructures it via match, the
linearity pass now emits a Severity::Warning diagnostic with a
form-A-rendered (borrow T) rewrite as suggested_rewrite. Pure
advisory, Decision 10 unchanged.

First Warning-severity diagnostic the typechecker emits; three
CLI exit paths gated on Error-only.

JOURNAL: design entry + iter 19a shipping entry.

crates/ail/src/main.rs

@@ -399,7 +399,16 @@ fn main() -> Result<()> {
                             d.message,
                         );
                     }
-                    std::process::exit(1);
+                    // Iter 19a: only Error-severity diagnostics fail the
+                    // command. Warning-severity diagnostics (e.g. the
+                    // `over-strict-mode` lint) print to stderr but the
+                    // module is still considered to have typechecked.
+                    if diags
+                        .iter()
+                        .any(|d| matches!(d.severity, ailang_check::Severity::Error))
+                    {
+                        std::process::exit(1);
+                    }
                 }
                 let total: usize = ws.modules.values().map(|m| m.defs.len()).sum();
                 println!(
@@ -430,7 +439,13 @@ fn main() -> Result<()> {
                         d.message,
                     );
                 }
-                std::process::exit(1);
+                // Iter 19a: only Error-severity blocks codegen.
+                if diags
+                    .iter()
+                    .any(|d| matches!(d.severity, ailang_check::Severity::Error))
+                {
+                    std::process::exit(1);
+                }
             }
             let ir = ailang_codegen::lower_workspace(&ws)?;
             match out {
@@ -1653,7 +1668,15 @@ fn build_to(
                 d.message,
             );
         }
-        std::process::exit(1);
+        // Iter 19a: only Error-severity blocks the build. Warning-
+        // severity diagnostics (e.g. `over-strict-mode`) print but
+        // do not abort.
+        if diags
+            .iter()
+            .any(|d| matches!(d.severity, ailang_check::Severity::Error))
+        {
+            std::process::exit(1);
+        }
     }
     // Iter 16b.3: run `lift_letrecs` per module on the post-desugar
     // form. Codegen's internal desugar pass is idempotent on a