test(ir): lock coercion forms before A4.3 extraction (A4.3 scaffolding step 1)

Test-first scaffolding ahead of extracting src/ir/conversions.zig — no code
change to the coercion targets (lowerXX / coerceToType / coerceOrErase /
buildProtocolErasure / tryUserConversion / failable-adapter selection).

Adds 4 .ir snapshots (first .ir for 01xx/09xx/10xx), each captured surgically
via `sx ir | normalize_ir`, path-free, idempotent, and print-free at IR-gen time
(0114-types-build-block-convert was rejected — it prints `--- void / 0 args ---`
+ sx source at IR-gen):
- 0107-types-int-cmp-in-float-ternary   numeric int<->float coercion
- 0903-optionals-optional-roundtrip     optional wrap/unwrap
- 0904-optionals-any-to-string-optional xx unbox_any + optional
- 1004-errors-try                       error-channel adapter/coercion

Protocol erasure + user Into are already pinned by the 04xx snapshots
(0400/0413/0414/0416); duplicate-conversion rejection by the 0410/0411/0412
anchors.

Adds 1 unit test via the public surface (no new exposure, mirroring A4.1/A4.2
sub-step 1): optionalOfFlattened — the optional wrap/flatten coercion rule
(T -> ?T; ?T -> ?T, never ??T; contrasted with the non-flattening optionalOf).
The lowerXX/coerceToType/coerceOrErase/buildProtocolErasure decisions are private
+ emission-bound, so their CoercionPlan unit tests land with the extracted module
in sub-step 2.

zig build, zig build test, tests/run_examples.sh (357/0) all green.

src/ir/lower.test.zig

@@ -879,3 +879,27 @@ test "protocols: registerProtocolDecl builds the dispatch method table" {
     try std.testing.expect(info.methods[1].ret_is_self);
     try std.testing.expectEqual(module.types.ptrTo(.void), info.methods[1].ret_type);
 }
+
+// ── A4.3 test-first scaffolding: coercion planning ───────────────────
+// Lock the one coercion-plan decision reachable via the existing public
+// surface — the optional wrap/flatten rule — before coercion planning moves to
+// `src/ir/conversions.zig`. The lowerXX / coerceToType / coerceOrErase /
+// buildProtocolErasure decisions are private + emission-bound, so their
+// CoercionPlan unit tests land with the extracted module in sub-step 2 (as the
+// generics/protocols plan tests landed with their modules); behavior is locked
+// here by the new `.ir` snapshots.
+
+test "conversions: optionalOfFlattened wraps once, flattening a nested optional" {
+    const alloc = std.testing.allocator;
+    var module = ir_mod.Module.init(alloc);
+    defer module.deinit();
+    var l = Lowering.init(&module);
+
+    const opt_s64 = module.types.optionalOf(.s64);
+    // Wrap a non-optional: T -> ?T.
+    try std.testing.expectEqual(opt_s64, l.optionalOfFlattened(.s64));
+    // Wrap an already-optional FLATTENS: ?T -> ?T (the coercion never builds ??T).
+    try std.testing.expectEqual(opt_s64, l.optionalOfFlattened(opt_s64));
+    // Contrast: the plain wrap does NOT flatten — ?T -> ??T (distinct type).
+    try std.testing.expect(module.types.optionalOf(opt_s64) != opt_s64);
+}