v0.29.0

This release updates the hegeltest-c dependency to 0.29.0.

libhegel C ABI

This release lets a caller redirect engine-emitted output (verbose / debug
progress traces and warnings) to a callback instead of stderr, by choosing the
destination per run or test case at creation
(#355).

hegel_run_start and hegel_test_case_from_blob each take a new
hegel_output_callback_t callback and void *user_data before the
out-parameter. The callback is invoked once per line of output, with
user_data passed through verbatim, so a binding can deliver engine output to
its own test logger (say, a Go testing.T). A NULL callback keeps the
output on stderr.

void deliver(void *user_data, const char *line, size_t len) { ... }

/* before */
hegel_run_start(ctx, settings, &run);
hegel_test_case_from_blob(ctx, settings, blob, &tc);

/* after */
hegel_run_start(ctx, settings, deliver, my_logger, &run);
hegel_test_case_from_blob(ctx, settings, blob, deliver, my_logger, &tc);

The destination is fixed when the run or test case is created — the engine
emits from its worker thread, and a run's output starts flowing the instant it
starts, so a per-call setter could not capture it without a race. For a run,
the callback (and whatever user_data points to) must stay valid until the run
is freed; for a blob replay, whose only line is emitted during the creating
call, it need not outlive that call. See the header documentation for the full
contract.