effects_coro coverage is 92.78% on Linux vs ~100% on macOS: forked death tests never flush gcov under GNU ld

[MelbourneDeveloper]

Symptom

_coverage_check_c_runtime measures effects_coro.c at 92.78% on Linux and materially higher on macOS. The gate threshold is therefore pinned at 91 (raised from main's 88) rather than the 95 the macOS numbers suggested.

Cause

compiler/runtime/test_death.h flushes coverage from the forked child with:

dlsym(RTLD_DEFAULT, "__gcov_dump")

On Mach-O this resolves. Under GNU ld it returns NULL unless the executable is linked with -rdynamic / -Wl,--export-dynamic, so every death test's coverage is silently discarded on Linux. The lines those tests exercise show as uncovered even though the assertions ran and passed.

Fix

Add -rdynamic (or -Wl,--export-dynamic) to the gcov-instrumented link in the Makefile, confirm dlsym resolves in a Debian container, then ratchet effects_coro back toward 95 once the Linux measurement supports it.

Verification

Reproduce in a container:

docker run --rm -v "$PWD":/w -w /w debian:bookworm bash -c '...gcc --coverage ... && gcov ...'

Compare effects_coro.c.gcov with and without -rdynamic.

Related: #197.