ERR/E1.3: raise sema + pure-failable lowering

`raise EXPR` now terminates a failable function via the error channel.
Scope (Option 2): full raise sema checks + lowering for the pure-failable
shape (`-> !` / `-> !Named`); the value-carrying `-> (T..., !)` shape bails
loudly, deferred to E2's error-channel tuple ABI.

- lowerStmt + tryLowerAsExpr: `.raise_stmt` -> lowerRaise (also routes a
  raise that is a block's last statement, which previously hit unknown_expr)
- lowerRaise: failable-context check (effectiveReturnType + errorChannelOf);
  literal membership via lowerErrorTagLiteral; variable form subset-checked
  via checkErrorSetSubset; pure-failable emits ret(tag)
- lowerErrorTagLiteral skips membership for the bare-`!` inferred placeholder
- plain `return;` in a pure-failable fn emits ret(0) (success / no error)
- parser: in_defer_body flag rejects `raise` inside a `defer` body

Tests: examples/219-raise.sx (positive, exit 8),
examples/220-raise-rejections.sx (3 sema rejections, exit 1), inline parser
test for raise-in-defer. Gates: zig build, zig build test, 258/258 examples.

examples/219-raise.sx

@@ -0,0 +1,26 @@
+// First runnable `raise` (ERR step E1.3). A `-> !Named` (pure failable)
+// function terminates via the error channel with `raise error.X`; a plain
+// `return;` is the success exit (error slot 0). The caller binds the result
+// and inspects it with the enum-like `==`. The value-carrying `-> (T, !)`
+// shape lands with the error-channel tuple ABI in ERR phase E2.
+
+#import "modules/std.sx";
+
+ParseErr :: error { BadDigit, Overflow, Empty }
+
+// Pure failable: raises on bad input, otherwise succeeds (error slot 0).
+check :: (n: s32) -> !ParseErr {
+    if n < 0 { raise error.BadDigit; }
+    return;                                 // success — no error
+}
+
+main :: () -> s32 {
+    good := check(7);                       // success path  -> no error
+    bad  := check(-1);                      // raise path    -> BadDigit
+    r : s32 = 0;
+    if bad  == error.BadDigit { r = r + 8; }    // true  -> +8
+    if good == error.BadDigit { r = r + 1; }     // false (success = no error)
+    if bad  == error.Overflow { r = r + 2; }     // false (raised BadDigit)
+    print("raise result: {}\n", r);              // -> 8
+    return r;
+}

examples/220-raise-rejections.sx

@@ -0,0 +1,32 @@
+// `raise` rejections (ERR step E1.3):
+//   - `raise` is only valid inside a failable function,
+//   - a literal `raise error.X` must name a tag in the function's set,
+//   - a variable `raise e` must carry a set that is a subset of the
+//     function's set.
+// The positive case lives in `examples/219-raise.sx`. Parse-time rejections
+// (`raise` in expression position / inside `defer` / `onfail`) are covered by
+// the inline parser tests.
+
+#import "modules/std.sx";
+
+ParseErr :: error { BadDigit, Overflow }
+OtherErr :: error { Weird }
+
+// Literal tag not in the declared set.
+bad_tag :: () -> !ParseErr {
+    raise error.NotInSet;            // error: NotInSet not in ParseErr
+}
+
+// Variable whose error set is not a subset of the function's set.
+makes_other :: () -> !OtherErr { return; }
+relay :: () -> !ParseErr {
+    e := makes_other();              // e : OtherErr
+    raise e;                         // error: OtherErr not subset of ParseErr
+}
+
+main :: () -> s32 {
+    x := bad_tag();                  // force bad_tag to lower
+    y := relay();                    // force relay to lower
+    raise error.BadDigit;            // error: main (-> s32) is not failable
+    return 0;
+}