ERR/E1.5: catch sema (pure-failable slice) + error-set match subjects

`expr catch [e] BODY` consumes a failable's error inline. Pure-failable slice
(value-carrying `-> (T, !)` catch deferred to E2's tuple ABI).

- lowerExpr `.catch_expr` -> lowerCatch; inferExprType `.catch_expr` ->
  operand's success type (void for pure-failable).
- lowerCatch: operand must be failable (else "catch requires a failable
  expression"); pure-failable LHS only (value-carrying bails to E2). Eval
  operand -> err tag; condBr to handle (error) / merge (success). In handle:
  child scope binds `e` to the tag (typed as the error set), lower body
  (block or expr); if the body didn't diverge, br merge. Result is void.
  `catch` needs no failable enclosing function — it handles the error locally.
- All four body forms work: block, no-binding `catch { }`, bare-expr, and
  the match-body `catch e == { case ... }`. Re-raise (`raise e`) and diverging
  bodies (`return`) rely on E1.3 / E1.4c.

Also: lowerMatch now supports error-set subjects — `case .X` resolves to the
global tag id (was the arm index, dispatching wrong), and the switch operand
is the error-set value (its u32 tag) directly rather than via enumTag. This
is what the catch match-body form (and a plain `if e == { case .X }`) needs.

Tests: examples/226-catch.sx (block / no-binding / match-body / re-raise /
diverging body / success-skip; exit 18), examples/227-catch-rejections.sx
(operand-not-failable; exit 1). Gates: zig build, zig build test,
265/265 examples.

tests/expected/226-catch.exit

@@ -0,0 +1 @@
+18

tests/expected/226-catch.txt

@@ -0,0 +1 @@
+catch result: 18

tests/expected/227-catch-rejections.exit

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

tests/expected/227-catch-rejections.txt

@@ -0,0 +1,5 @@
+error: `catch` requires a failable expression; operand has type 's32'
+  --> /Users/agra/projects/sx/examples/227-catch-rejections.sx:11:5
+   |
+11 |     plain() catch e { return 1; };   // error: operand has type s32 (not failable)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^