Adds `make coverage` (per-package + total via -coverpkg=./...),
`make coverage-html`, and `make coverage-total` (CI-friendly). Wires
coverage.out/coverage.html through `make clean` and .gitignore.
Closes the two easy zero-coverage packages: namegen (77.8%) and
sessionstream (93.5%). Total statement coverage 51.7% -> 52.1%.
