chore: give every coverage pragma a reason (TS6)

[lesnik512]

Brings the coverage pragmas in line with TS6; TS5 exclude_also was already compliant.

Counts: 0 tested / 0 deleted / 6 reasoned.

• fixer.py _assert_never and the case _: arm: kept, per ADR-0002 (the vendored guard pins the ty error at the match arm). Reason names the static check.
• fixer.py trailing raise AssertionError("unreachable"): kept; after the loop remaining is a non-empty run of \n/\r, so one of the three sequences always matches and returns.
• _no_subprocess in test_discovery.py: runs only if discovery spawns a subprocess, which is what the test forbids.
• The two symlink except (OSError, NotImplementedError) skips: taken only where the OS refuses symlinks; CI runs on Linux.

Local checks: just install, just lint-ci (eof-fixer, ruff format, ruff check, ty) pass; just test-ci 44 passed, 100 % coverage.