issues: promote 6 fixed bug repros to focused regression tests

All six produce their target outputs cleanly today; renamed out of
the `issue-*` namespace per CLAUDE.md "Resolving an open issue":

| Old                  | New                                       |
|----------------------|-------------------------------------------|
| issue-0032           | 181-impl-duplicate-same-file              |
| issue-0041           | 182-compound-type-in-expression           |
| issue-0042           | 183-type-alias-size-align                 |
| issue-0044           | 184-objc-defined-class-method-self        |
| issue-0045           | 185-pack-fn-comptime-return               |
| issue-0046           | 186-nested-comptime-return                |

Comment headers tightened to feature-focused (drop the issue-NNNN
provenance — that's in git history now). Missing expected `.txt` /
`.exit` files captured for 0041 + 0042 (they were untracked because
the bugs were fixed silently in adjacent work).

`examples/issue-*` after this commit: just `issue-0030.sx` — a
feature request (`extern G : T;` cross-file globals) that's never
been implemented. Staying in the issue namespace as a parked
proposal until the feature lands or gets formally rejected.

220/220 example tests + `zig build test` green.

tests/expected/181-impl-duplicate-same-file.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/181-impl-duplicate-same-file.sx:15:1: error: duplicate impl 'Into' for source 's64' in /Users/agra/projects/sx/examples/181-impl-duplicate-same-file.sx

tests/expected/182-compound-type-in-expression.txt

@@ -0,0 +1,10 @@
+size_of(*u8)        = 8
+align_of(*u8)       = 8
+size_of(?u8)        = 2
+size_of([3]u8)      = 3
+size_of((s32)->s32) = 8
+size_of((s32, s32)) = 8
+size_of(Ptr)        = 8
+size_of(Maybe)      = 2
+size_of(Arr)        = 3
+size_of(Cb)         = 8

tests/expected/183-type-alias-size-align.txt

@@ -0,0 +1,7 @@
+direct s32:        4
+alias s32:         4
+chain s32:         4
+align alias:       4
+align chain:       4
+size struct-alias: 16
+align struct-alias:8

tests/expected/185-pack-fn-comptime-return.exit

@@ -0,0 +1 @@
+0

tests/expected/186-nested-comptime-return.exit

@@ -0,0 +1 @@
+0

tests/expected/issue-0032.txt

@@ -1 +0,0 @@
-/Users/agra/projects/sx/examples/issue-0032.sx:13:1: error: duplicate impl 'Into' for source 's64' in /Users/agra/projects/sx/examples/issue-0032.sx