ERR/E1.4b: whole-program inferred error sets + empty-inferred warning

The type-convergence side of E1.4 (the SCC slice). A bare `-> !` function's
error set is now converged whole-program from its literal raises plus the
sets of the pure-failable functions it `try`s.

- convergeInferredErrorSets: a pre-lowering fix-point pass (lowerRoot Pass
  1d, after scanDecls / before body lowering) that walks each top-level
  bare-`!` function's body AST (collectErrorSites, stopping at nested-fn
  boundaries) for literal `raise error.X` tags + pure `try g()` edges, then
  unions each set with its edges' sets until stable. Stored in a side map
  `inferred_error_sets` (fn name -> sorted []u32) — sidesteps the name-only
  error-set interning collision (the shared `!` placeholder stays empty).
- lowerTry widening: a named caller `try`-ing a bare-`!` callee now checks
  the callee's converged set (previously a false-negative — the empty
  placeholder was trivially a subset). Factored diagTagsNotInSet out of
  checkErrorSetSubset.
- empty-inferred warning: a top-level non-main bare-`!` function with an
  empty converged set warns. Not user-visible yet (the compile driver
  renders diagnostics only on failure — a LANG follow-up), so unit-tested
  on the DiagnosticList.
- corrected two now-stale bail messages (failable-`or` -> E2.4;
  value-carrying `try` -> E2).

Deferred to E2.4: failable-`or` chains / value-terminators (and `try`
fallback routing) — gated on the value-carrying tuple ABI.

Tests: examples/223-inferred-error-sets.sx (transitive convergence +
widening passes, exit 7), examples/224-inferred-widening-reject.sx
(transitive widening rejection, exit 1), unit test in lower.test.zig.
Gates: zig build, zig build test, 262/262 examples.

examples/223-inferred-error-sets.sx

@@ -0,0 +1,40 @@
+// Whole-program inferred error sets (ERR step E1.4b). A bare `-> !` function's
+// error set is INFERRED: the union of the tags it raises directly plus the
+// sets of the failable functions it `try`s, converged across the whole call
+// graph by a fix-point pass. Here `leaf` raises {Foo}; `mid` try-propagates
+// leaf AND raises Bar, so `mid` converges to {Foo, Bar}; the named caller
+// `run :: -> !A` then type-checks because mid's converged set is a subset of
+// A. The rejection (a converged tag NOT in the caller's set) lives in
+// `examples/224-inferred-widening-reject.sx`.
+
+#import "modules/std.sx";
+
+A :: error { Foo, Bar }
+
+leaf :: (n: s32) -> ! {
+    if n < 0 { raise error.Foo; }
+    return;
+}
+
+// Inferred set converges to {Foo, Bar}: {Foo} absorbed from `try leaf` plus
+// the directly-raised Bar.
+mid :: (n: s32) -> ! {
+    try leaf(n);
+    if n == 100 { raise error.Bar; }
+    return;
+}
+
+// Named caller: mid's converged {Foo, Bar} is a subset of A -> widening OK.
+run :: (n: s32) -> !A {
+    try mid(n);
+    return;
+}
+
+main :: () -> s32 {
+    e := run(-1);                          // leaf raises Foo -> propagates out
+    r : s32 = 0;
+    if e == error.Foo { r = r + 7; }       // true  -> +7
+    if e == error.Bar { r = r + 1; }        // false (Foo escaped, not Bar)
+    print("inferred result: {}\n", r);      // -> 7
+    return r;
+}

examples/224-inferred-widening-reject.sx

@@ -0,0 +1,30 @@
+// Inferred-set widening rejection (ERR step E1.4b). When a named caller
+// (`-> !A`) `try`s a bare-`!` callee, the callee's WHOLE-PROGRAM-CONVERGED
+// inferred set must be a subset of A. Before the SCC pass this was a
+// false-negative (the bare-`!` placeholder was empty, so the check trivially
+// passed); now the converged tags are checked. `deep`'s converged set is
+// {Foo} (raised transitively through `via`), which is not in A = {Bar}.
+// The positive case lives in `examples/223-inferred-error-sets.sx`.
+
+#import "modules/std.sx";
+
+A :: error { Bar }
+
+deep :: () -> ! {
+    raise error.Foo;                 // deep's inferred set = {Foo}
+}
+
+via :: () -> ! {
+    try deep();                      // via absorbs {Foo}
+    return;
+}
+
+caller :: () -> !A {
+    try via();                       // error: Foo (via's converged set) not in A
+    return;
+}
+
+main :: () -> s32 {
+    e := caller();
+    return 0;
+}