refactor: canonical failable syntax (T, !) — remove the bare -> T ! sugar

The trailing-`!`-after-the-value-type spelling (`-> T !`, `-> Tuple(A,B) !`) was a
redundant second way to write a failable return that the parser folded into the
same AST as the parenthesized `(T, !)` / `(A, B, !)` result list. Remove it so
there is ONE canonical spelling: the error channel always rides as the last slot
of the parenthesized list.

- parser: `parseFnReturnType` no longer folds a trailing `!` after a value type —
  it rejects it with a located diagnostic ("a failable return is written `(T, !)`
  … not `T !`"). This one chokepoint covers fn declarations, lambdas, fn-pointer
  types `(A) -> R`, and closure types `Closure(A) -> R`. The error-ONLY `-> !` /
  `-> !ErrSet` form is unaffected (parsed by parseTypeExpr as an error_type_expr).
- migrated every usage to canonical form across library/ + examples/ + issues/ +
  tests/: `-> T !E` → `-> (T, !E)`; the value-carrying `-> Tuple(A, B) !` (which
  FLATTENED to a multi-value failable) → `-> (A, B, !)`, preserving behavior. A
  genuine single-tuple-value failable stays `-> (Tuple(A,B), !)`.
- parser unit tests: the "bare form folds" tests become "bare form is rejected";
  canonical-form parse tests retained.
- docs: specs.md §12 + scattered refs and readme.md updated to the `(T, !)` form.

Behavior-preserving (the bare form was sugar for the same AST). Adversarial review
confirmed: rejection complete across all positions, every canonical form works on
both success/error paths, error-only `-> !` intact, no crashes. Full suite green
(unit tests + 850 corpus examples).

library/vendors/sqlite/sqlite.sx

@@ -461,7 +461,7 @@ SqliteStmt :: struct {
     // ── execution ──
     // SQLITE_ROW / SQLITE_DONE on success; anything else raises with the
     // detail left in the connection's errmsg.
-    step :: (self: *SqliteStmt) -> i32 !SqliteErr {
+    step :: (self: *SqliteStmt) -> (i32, !SqliteErr) {
         rc := sqlite3_step(self.handle);
         if rc != SQLITE_ROW and rc != SQLITE_DONE { raise error.Step; }
         return rc;
@@ -564,7 +564,7 @@ ColumnMeta :: struct {
 Sqlite :: struct {
     handle: usize;
 
-    open :: (path: string) -> Sqlite !SqliteErr {
+    open :: (path: string) -> (Sqlite, !SqliteErr) {
         h : usize = 0;
         rc := sqlite3_open(to_cstring(path), @h);
         if rc != SQLITE_OK {
@@ -574,7 +574,7 @@ Sqlite :: struct {
         return Sqlite.{ handle = h };
     }
 
-    open_v2 :: (path: string, flags: i32) -> Sqlite !SqliteErr {
+    open_v2 :: (path: string, flags: i32) -> (Sqlite, !SqliteErr) {
         h : usize = 0;
         rc := sqlite3_open_v2(to_cstring(path), @h, flags, 0);
         if rc != SQLITE_OK {
@@ -603,7 +603,7 @@ Sqlite :: struct {
         return;
     }
 
-    prepare :: (self: *Sqlite, sql: string) -> SqliteStmt !SqliteErr {
+    prepare :: (self: *Sqlite, sql: string) -> (SqliteStmt, !SqliteErr) {
         sh : usize = 0;
         rc := sqlite3_prepare_v2(self.handle, sql.ptr, xx sql.len, @sh, 0);
         if rc != SQLITE_OK { raise error.Prepare; }
@@ -611,7 +611,7 @@ Sqlite :: struct {
     }
     // prepare with SQLITE_PREPARE_* flags (e.g. PERSISTENT for the
     // statement cache a storage layer keeps hot).
-    prepare_v3 :: (self: *Sqlite, sql: string, flags: u32) -> SqliteStmt !SqliteErr {
+    prepare_v3 :: (self: *Sqlite, sql: string, flags: u32) -> (SqliteStmt, !SqliteErr) {
         sh : usize = 0;
         rc := sqlite3_prepare_v3(self.handle, sql.ptr, xx sql.len, flags, @sh, 0);
         if rc != SQLITE_OK { raise error.Prepare; }
@@ -685,7 +685,7 @@ Sqlite :: struct {
     }
 
     // Schema introspection for one column of "main".`table`.
-    table_column_metadata :: (self: *Sqlite, table: string, column: string) -> ColumnMeta !SqliteErr {
+    table_column_metadata :: (self: *Sqlite, table: string, column: string) -> (ColumnMeta, !SqliteErr) {
         dt : usize = 0;
         cs : usize = 0;
         nn : i32 = 0;
@@ -703,7 +703,7 @@ Sqlite :: struct {
 
     // ── serialization ──
     // The whole "main" database as bytes (a valid database image).
-    serialize :: (self: *Sqlite) -> string !SqliteErr {
+    serialize :: (self: *Sqlite) -> (string, !SqliteErr) {
         size : i64 = 0;
         p := sqlite3_serialize(self.handle, to_cstring("main"), @size, 0);
         if p == null { raise error.Serialize; }
@@ -734,7 +734,7 @@ Sqlite :: struct {
 SqliteBlob :: struct {
     handle: usize;
 
-    open :: (db: *Sqlite, table: string, column: string, rowid: i64, writable: bool) -> SqliteBlob !SqliteErr {
+    open :: (db: *Sqlite, table: string, column: string, rowid: i64, writable: bool) -> (SqliteBlob, !SqliteErr) {
         h : usize = 0;
         rc := sqlite3_blob_open(db.handle, to_cstring("main"), to_cstring(table), to_cstring(column),
                                 rowid, if writable then 1 else 0, @h);
@@ -751,7 +751,7 @@ SqliteBlob :: struct {
     bytes :: (self: *SqliteBlob) -> i32 {
         return sqlite3_blob_bytes(self.handle);
     }
-    read :: (self: *SqliteBlob, offset: i32, n: i32) -> string !SqliteErr {
+    read :: (self: *SqliteBlob, offset: i32, n: i32) -> (string, !SqliteErr) {
         len : i64 = n;
         raw : [*]u8 = xx context.allocator.alloc_bytes(len + 1);
         rc := sqlite3_blob_read(self.handle, raw, n, offset);
@@ -778,7 +778,7 @@ SqliteBlob :: struct {
 SqliteBackup :: struct {
     handle: usize;
 
-    init :: (dst: *Sqlite, src: *Sqlite) -> SqliteBackup !SqliteErr {
+    init :: (dst: *Sqlite, src: *Sqlite) -> (SqliteBackup, !SqliteErr) {
         h := sqlite3_backup_init(dst.handle, to_cstring("main"), src.handle, to_cstring("main"));
         if h == 0 { raise error.Backup; }
         return SqliteBackup.{ handle = h };