system.timezone does not observe /etc/timezone

[haribo]

Part of #634.

system.timezone's apply writes two things:

ln -sf "/usr/share/zoneinfo/$zone" /etc/localtime
printf '%s\n' "$zone" > /etc/timezone

Its observe reads one:

readlink -f /etc/localtime | sed 's#^/usr/share/zoneinfo/##'

A host whose /etc/localtime is right and whose /etc/timezone is wrong — or missing — is
reported converged, and the file stays wrong. That file is what dpkg-reconfigure tzdata,
tzupdate and several distro tools read, so it is not decorative.

The def already carries a note about why it observes the link rather than date +%Z
(seasonal abbreviations). It says nothing about the second file.

Build

Observe both, or stop writing the second one. Observing both is the smaller claim and
matches the apply: state(zone: <link>, recorded: <file contents>).

Worth settling in passing: on a host where /etc/timezone does not exist at all (not every
distribution ships it), writing it is either right or noise — the def's comment about
dpkg-reconfigure suggests Debian is the assumption, and this is a good place to say so.

Validation

• An adverse case: correct link, /etc/timezone holding another zone, then the def, then
  assert the file's contents on the machine. Red first.
• coverage.shellf's existing call still converges on a second run.

[haribo]

Audit against the current code

Central claim: confirmed.

• internal/std/system/timezone.shellf:28-29 — the apply writes two things: the
  /etc/localtime symlink and /etc/timezone.
• internal/std/system/timezone.shellf:24 — the observe returns one field, zone, read from
  readlink -f /etc/localtime. /etc/timezone is never read.
• Consequence: zone is named after the parameter zone, so internal/lang/eval.go:471-489
  compares it to the argument. A host whose link is right and whose /etc/timezone is wrong
  or missing is reported converged, and the file stays wrong.

Two corrections.

1. This issue says the def "says nothing about the second file". It does —
   internal/std/system/timezone.shellf:14-15 explains why /etc/timezone is written
   (Debian's tooling reads it; a host where the two disagree is one where
   dpkg-reconfigure tzdata silently undoes the def). What is undocumented is why the
   observe ignores a file the def considers important enough to write.

2. The proposed shape does not work. state(zone: <link>, recorded: <file contents>)
   would converge over any non-empty content: recorded has no same-named parameter, so
   converged (eval.go:471-483) only requires it to be truthy, and Europe/Berlin is
   truthy. The field must be a comparison, in the shape the def's neighbours use:
   recorded: shell { [ "$(cat /etc/timezone 2>/dev/null)" = "$zone" ] }.exit == 0.

The existing adverse case does not catch it, for the same reason as #635.
test/e2e/plans/adverse-system.timezone.shellf:9 breaks both the link and the file, so the
apply runs; :13 then asserts /etc/timezone and passes. The uncovered case is the one where
the observe converges: link right, file wrong.

The open question, settled. printf '%s\n' "$zone" > /etc/timezone creates the file on a
host that does not ship one, and the added field reads it back — so a non-Debian host
converges on the second run rather than acting forever. Worth one line in the def's header,
not a behaviour change.

[haribo]

Delivered in #644, squashed to e30fe0a on develop.

internal/std/system/timezone.shellf now observes state(zone: …, recorded: …). Not the
shape this issue proposed: recorded is a comparison, because a field with no same-named
parameter only has to be truthy (internal/lang/eval.go:471-483) and the file's contents would
have converged whatever zone they named.

The open question is settled in the def's comment: on a host shipping no /etc/timezone, the
comparison fails, the apply creates the file, and the next run converges — no Debian assumption
needed.

test/e2e/plans/adverse-system.timezone-recorded.shellf, verified red first:

RED   system.timezone(zone=Europe/Paris) ok.already   → /etc/timezone still Europe/Berlin
green system.timezone(zone=Europe/Paris) ok.set       → Europe/Paris

Green in CI's e2e job.