ERR/E2.4b: failable or chains

`lhs or rhs` with failable operands now lowers as a full short-circuit
chain (was a loud bail). Each failing attempt routes to the next operand;
the chain resolves when an operand succeeds or a value terminator absorbs;
total failure propagates to the function — or, when the chain is the operand
of a `catch`, to the handler. All in ir/lower.zig.

- Dispatch (lowerBinaryOp .or_op): structural `orIsFailableChain` (an operand
  is a `try`, error-channel-typed, or a nested failable `or` chain) instead of
  the type-only `exprIsFailable(lhs)`, which missed nested chains (a try-chain's
  value type is non-failable T).
- inferExprType .or_op: a failable chain reports its success type via
  `orChainSuccessType` (was `.bool`).
- lowerFailableOr rewritten: flatten the left-assoc chain, lower operands
  left-to-right. Non-final failure → push frame + fall to next operand block
  (no function exit, so onfail doesn't fire). Success → clear trace + merge.
  Final failure → push frame + route to a `catch` target (chain_fail_target
  field) if set, else propagate (cleanup + error return). Value terminator →
  clear + merge the terminator value. Subsumes the E2.4a path. Widening
  factored into `checkEscapeWidening`, checked only at a propagating final
  operand.
- Catch-over-chain: lowerCatchOverChain sets chain_fail_target so the chain's
  total failure reaches the handler (binds the final tag, may inspect the
  trace, clears on non-diverging exit).

Verified JIT + AOT: 2-/3-operand chains, bare chain + value terminator, void
chains, all-fail propagation (exit 1 + trace), catch-over-chain, trace
clear-on-absorb, onfail gating. examples/246-failable-or-chain.sx (exit 120),
247-failable-or-chain-propagate.sx (exit 1 + trace).

tests/expected/246-failable-or-chain.exit

@@ -0,0 +1 @@
+120

tests/expected/246-failable-or-chain.txt

@@ -0,0 +1 @@
+

tests/expected/247-failable-or-chain-propagate.exit

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

tests/expected/247-failable-or-chain-propagate.txt

@@ -0,0 +1,8 @@
+error: unhandled error reached main: error.A
+error return trace (most recent call last):
+  frame 0: <location pending DWARF> (raw 1)
+  frame 1: <location pending DWARF> (raw 1)
+  frame 2: <location pending DWARF> (raw 1)
+  frame 3: <location pending DWARF> (raw 1)
+  frame 4: <location pending DWARF> (raw 1)
+  frame 5: <location pending DWARF> (raw 1)