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.

tests/expected/219-raise.exit

@@ -0,0 +1 @@
+8

tests/expected/219-raise.txt

@@ -0,0 +1 @@
+raise result: 8

tests/expected/220-raise-rejections.exit

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

tests/expected/220-raise-rejections.txt

@@ -0,0 +1,17 @@
+error: error tag 'error.NotInSet' is not in error set 'ParseErr'
+  --> /Users/agra/projects/sx/examples/220-raise-rejections.sx:17:11
+   |
+17 |     raise error.NotInSet;            // error: NotInSet not in ParseErr
+   |           ^^^^^^^^^^^^^^
+
+error: error tag 'error.Weird' is not in caller's error set 'ParseErr'
+  --> /Users/agra/projects/sx/examples/220-raise-rejections.sx:24:5
+   |
+24 |     raise e;                         // error: OtherErr not subset of ParseErr
+   |     ^^^^^^^^
+
+error: `raise` is only valid inside a failable function (a return type with `!` or `!Named`)
+  --> /Users/agra/projects/sx/examples/220-raise-rejections.sx:30:5
+   |
+30 |     raise error.BadDigit;            // error: main (-> s32) is not failable
+   |     ^^^^^^^^^^^^^^^^^^^^^