ERR/E1.4a: standalone try sema + pure-failable propagation + named widening

`try f()` (standalone form) now propagates a failable callee's error to the
enclosing failable function. E1.4 was split: E1.4a = standalone try (failure
target = function-propagation); E1.4b = fallback-target routing +
failable-`or` + whole-program SCC for inferred sets + empty-inferred warning.

- lowerExpr: `.try_expr` -> lowerTry
- lowerTry: (1) try legal only inside a failable fn; (2) the sole
  failable-operand check (errorChannelOf(inferExprType(operand))); (3)
  named-caller widening (checkErrorSetSubset at the propagation site); (4)
  pure-failable lowering — condBr on tag != 0: propagate (run defers + ret
  the widened tag) vs continue on success
- inferExprType: `.try_expr` arm (success type: void for pure-failable)
- lowerBinaryOp .or_op: bail loudly on a failable LHS (exprIsFailable);
  the optional-`or` path is unchanged for non-failable LHS
- value-carrying callee/caller `try` bail loudly (pending E2's tuple ABI)

Tests: examples/221-try.sx (positive propagation, exit 5),
examples/222-try-rejections.sx (3 stable rejections: outside-failable,
non-failable operand, named-widening miss; exit 1). Gates: zig build,
zig build test, 260/260 examples.

tests/expected/221-try.exit

@@ -0,0 +1 @@
+5

tests/expected/221-try.txt

@@ -0,0 +1 @@
+try result: 5

tests/expected/222-try-rejections.exit

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

tests/expected/222-try-rejections.txt

@@ -0,0 +1,17 @@
+error: `try` is only valid inside a failable function (a return type with `!` or `!Named`)
+  --> /Users/agra/projects/sx/examples/222-try-rejections.sx:20:5
+   |
+20 |     try ga();                  // error: `try` outside a failable function
+   |     ^^^^^^^^
+
+error: `try` requires a failable expression; operand has type 's32'
+  --> /Users/agra/projects/sx/examples/222-try-rejections.sx:26:5
+   |
+26 |     try plain();               // error: operand has type s32 (not failable)
+   |     ^^^^^^^^^^^
+
+error: error tag 'error.Yb' is not in caller's error set 'A'
+  --> /Users/agra/projects/sx/examples/222-try-rejections.sx:32:5
+   |
+32 |     try gb();                  // error: Yb not in caller's error set A
+   |     ^^^^^^^^