Escape propagation: try-swallows-exit and grant tail-call bypass

Two sibling bugs in how non-local escapes propagated through scope boundaries, fixed together and pinned by regression tests in core/tests/scope_escapes.rs (try_does_not_swallow_exit, grant_attenuates_across_tail_recursion).

try swallows exit. try caught more than it should: a genuine exit escaping through a try body was absorbed as if it were a recoverable failure, so the process did not terminate. An escape carrying program exit must travel through try, not be unified into its error record.

grant tail-call bypass. A tail call made from inside a grant body could land outside the grant’s dynamic frame, evaluating with authority the grant was supposed to have removed. Capability frames must stay in force across tail landings, exactly as scoping requires.

Both are corrected; the regression tests are the contract. The escape surface that makes this regular is recorded in completion-escape-refactor.

See also control-operators, grant.