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`onfail [e] BODY` runs cleanup only when an error LEAVES the enclosing block
(a `raise` or a propagating `try`), and is skipped on success — unlike `defer`,
which runs on every exit. On an error exit, defers and onfails run interleaved
in reverse declaration order; `onfail e` binds the in-flight error tag.
- Cleanup stack: defer_stack now holds CleanupEntry { body, is_onfail, binding }
(one declaration-ordered stack so defer/onfail interleave). lowerDefer pushes
a defer entry; lowerOnFail (new `.onfail_stmt` arm) pushes an onfail entry,
rejecting `onfail` outside a failable function.
- emitBlockDefers (success exits — return / normal block exit) now emits only
`defer` entries and discards onfails.
- emitErrorCleanup (new; wired at the error exits — lowerRaise pure +
value-carrying, lowerTry propagation) emits both kinds interleaved in reverse,
binding the in-flight tag for `onfail e`.
Block-rooted: an error propagating to the function drains all enclosing blocks'
onfails; a block that exits normally discards its onfails. Per-attempt-`try`
gating is moot for now (no compilable `or` chain can absorb a mid-block try
failure yet — E2.4b). Body restrictions beyond the parser's raise-in-onfail
ban are deferred.
Tests: examples/233-onfail.sx (interleave order on error vs success + binding;
deterministic trace), examples/234-onfail-reject.sx (onfail outside a failable
fn rejected; exit 1). Gates: zig build, zig build test, 272/272 examples.
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