generators: document TOML date limitation in mkImpureConfigMerger

jaq 3.1.1 reads TOML through toml-span, which does not deserialize
date or time values. An existing TOML file containing one makes the
merge refuse, so activation fails on every switch until the value is
removed. The file itself is left untouched.

Document the limitation and test that the refused merge leaves the
file byte-identical. If a later jaq parses these values, they would
pass through JSON as strings, so the test is expected to fail then and
prompt a review of the documented behavior.

modules/lib/generators.nix

@@ -930,6 +930,10 @@ in
     preserved. YAML is written in block style with `---` and `...` document
     markers.
 
+    As of `jaq` 3.1.1, TOML date and time values are not supported. An existing
+    TOML file containing such values causes activation to fail without modifying
+    the file. Subsequent activations will also fail while those values remain.
+
     :::{.warning}
     This function is **experimental**: its interface and generated script may
     change without notice in future releases, as edge cases around the

tests/lib/generators/mkImpureConfigMerger.nix

@@ -102,6 +102,12 @@ let
       key: nix
     ...
   '';
+
+  userDateToml = builtins.toFile "user-date.toml" ''
+    # Keep this file intact when jaq cannot parse its date value.
+    date = 1979-05-27T07:32:00Z
+    userSetting = 5
+  '';
 in
 {
   nmt.script = ''
@@ -178,6 +184,15 @@ in
     $TMPDIR/merge-toml
     assertFileContent $tomlSettings ${expectedToml}
 
+    # jaq 3.1.1 cannot parse TOML dates. If a newer jaq can, they would pass
+    # through JSON as strings and come back as TOML strings, so this failing
+    # means the documented limitation needs revisiting.
+    cat ${userDateToml} > $tomlSettings
+    if $TMPDIR/merge-toml > /dev/null 2>&1; then
+      fail "Merging TOML with a date value must fail activation"
+    fi
+    assertFileContent $tomlSettings ${userDateToml}
+
     substitute ${mergeYaml} $TMPDIR/merge-yaml --subst-var TMPDIR
     chmod +x $TMPDIR/merge-yaml
     yamlSettings=$TMPDIR/hm-user/.config/testmerger/settings.yaml