Add cbm_str_copy() as the shared allocation-free fixed-buffer copy helper and use it in httplink, envscan, and store file-tree code.
Remove guarded production strncpy() sites, add a source-safety rule and self-test for future strncpy regressions, and tighten the logging thread-safety comment to match the implementation.
Validation: make -f Makefile.cbm lint-source-safety; CBM_ONLY_SUITE=httplink make -f Makefile.cbm test; CBM_ONLY_SUITE=str_util build/c/test-runner; CBM_ONLY_SUITE=pipeline CBM_ONLY_TEST=envscan build/c/test-runner; CBM_ONLY_SUITE=store_arch build/c/test-runner.
Signed-off-by: Andrew Hundt <ATHundt@gmail.com>