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.

tests/expected/223-inferred-error-sets.exit

@@ -0,0 +1 @@
+7

tests/expected/223-inferred-error-sets.txt

@@ -0,0 +1 @@
+inferred result: 7

tests/expected/224-inferred-widening-reject.exit

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

tests/expected/224-inferred-widening-reject.txt

@@ -0,0 +1,5 @@
+error: error tag 'error.Foo' is not in caller's error set 'A'
+  --> /Users/agra/projects/sx/examples/224-inferred-widening-reject.sx:23:5
+   |
+23 |     try via();                       // error: Foo (via's converged set) not in A
+   |     ^^^^^^^^^