fix(cowsay): resolve doctor/verify probe at /usr/games packaged path

[ycpss91255]

The bug

The Debian/Ubuntu cowsay package installs its binary at /usr/games/cowsay,
and /usr/games is absent from the default non-login PATH used by bash -c,
docker exec ... bash -c, and cron. Both cowsay health probes used a bare
command -v cowsay:

• TEST_VERIFY_CMD="command -v cowsay" (consumed by verify via
  module_default_verify -> bash -c "${TEST_VERIFY_CMD}")
• the doctor() override

So on a correctly-installed cowsay, doctor and verify FALSELY reported
failure while dpkg-based is_installed still reported installed -- an
internally inconsistent installed=yes / doctor=fail state.

Scope: only cowsay installs under /usr/games among landed modules.
cmatrix (/usr/bin/cmatrix) and figlet (/usr/bin/figlet) are on the
default PATH and are unaffected -- untouched.

The fix

Both probes now try PATH first, then fall back to the packaged path:

command -v cowsay >/dev/null 2>&1 || [ -x "${COWSAY_GAMES_BIN:-/usr/games/cowsay}" ]

• The command -v branch stays first, so a hypothetical PATH-resolvable
  cowsay still wins.
• COWSAY_GAMES_BIN overrides the packaged path (default /usr/games/cowsay),
  matching the repo's established ${BIN_LINK:-/usr/local/bin/...} seam
  convention, and lets unit tests inject a stub instead of writing to the
  container global filesystem.
• The doctor() warning now names both places it checked (PATH and the
  packaged path).

The unit spec previously MASKED the bug by faking cowsay onto PATH via the
_fake_bin helper, so it never exercised the real /usr/games location. It
now uses a _fake_games_cowsay helper + COWSAY_GAMES_BIN to simulate the
packaged location and asserts doctor + verify pass when cowsay exists ONLY
there, and still fail when absent from both PATH and /usr/games. A separate
test keeps the PATH-resolvable branch (command -v) covered.

RED-then-GREEN test evidence

RED (new tests against the unmodified module -- the "only at /usr/games"
assertions fail while the absent-case assertions pass):

not ok 32 verify passes via real probe when cowsay exists only at /usr/games
ok 33 verify fails via real probe when cowsay is absent from PATH and /usr/games
not ok 36 doctor passes when cowsay exists only at /usr/games, absent from PATH
ok 37 doctor fails when cowsay is absent from both PATH and /usr/games

GREEN (after the module fix -- just -f justfile.ci test-unit cowsay, all 54
pass):

ok 31 verify passes when installed and TEST_VERIFY_CMD succeeds
ok 32 verify passes via real probe when cowsay exists only at /usr/games
ok 33 verify fails via real probe when cowsay is absent from PATH and /usr/games
ok 34 doctor fails when not installed
ok 35 doctor passes when cowsay resolves on PATH (command -v branch)
ok 36 doctor passes when cowsay exists only at /usr/games, absent from PATH
ok 37 doctor fails when cowsay is absent from both PATH and /usr/games

Gates (Docker, blocking foreground)

• just -f justfile.ci lint -> ShellCheck OK, fish OK, Hadolint OK
• just -f justfile.ci test-unit -> exit 0 (full suite)
• just -f justfile.ci test-integration -> exit 0 (24/24 ok)

Provenance

Found by an adversarial code-review workflow. doc/changelog/CHANGELOG.md
gains an [Unreleased] > Fixed entry. No module METADATA changed, so
doc/module/INDEX.md needs no regeneration.