ArgParser: read ArgumentHelpText from nested records and unions (#602)

The type-level help lookup ran only against the [<ArgParser>]-tagged
root, so an [<ArgumentHelpText>] on a nested argument record or union
was silently ignored, despite the attribute documenting itself as
applying to a record type generally. RecordType.Attributes and
UnionType.Attributes were already to hand at the nesting site; nothing
was reading them.

The nested type's help now heads the group its arguments form, wherever
that type is embedded. A field's own [<ArgumentHelpText>] overrides it:
the field is the more specific placement.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Smaug123

CHANGELOG.md

@@ -1,5 +1,13 @@
 Notable changes are recorded here.
 
+# WoofWare.Myriad.Plugins 10.7.2
+
+`ArgParserGenerator` now reads `[<ArgumentHelpText>]` from a nested argument record or union of alternative argument sets, and not only from the `[<ArgParser>]`-tagged root, and from a discriminated union's case (and that case's payload record).
+The nested type's help heads the group of arguments that type contributes, wherever it is embedded, and a case's help heads its group the same way.
+
+An `[<ArgumentHelpText>]` on the more specific placement overrides the more general one: a field's overrides its nested type's, and a case's overrides its payload record's.
+One type, or one payload record, may be embedded or reused at several sites for different purposes, so the more specific placement is the one which can say what a particular occurrence is for.
+
 # WoofWare.Myriad.Plugins 10.7.1
 
 `ArgParserGenerator` help text now groups the arguments contributed by a field whose type is another argument record, or a union of alternative argument sets, under a header line naming that field.

ConsumePlugin/Args.fs

@@ -158,6 +158,36 @@ type ParentRecordWithGroupHelp =
         AndAnother : bool
     }
 
+/// A nested type may describe itself, for the benefit of every site which embeds it.
+[<ArgumentHelpText "How to talk to the database">]
+type DescribedChild =
+    {
+        Host : string
+        Port : int
+    }
+
+/// `Primary` takes the type's own description; `Secondary` overrides it, because the field is the
+/// more specific placement and one type may be embedded for different purposes.
+[<ArgParser true>]
+type ParentRecordWithTypeHelp =
+    {
+        [<ArgumentPrefix "primary">]
+        Primary : DescribedChild
+        [<ArgumentPrefix "secondary">]
+        [<ArgumentHelpText "Where to fail over to">]
+        Secondary : DescribedChild
+    }
+
+/// Help text may contain characters which need escaping to survive being reproduced in the
+/// generated file: FCS decodes the literal before Myriad ever sees it, so a backslash, a quote,
+/// and a control character must all be re-escaped rather than passed through as the decoded text.
+[<ArgParser true>]
+type ParentRecordWithEscapedHelp =
+    {
+        [<ArgumentHelpText "Path is C:\\temp, quote is \" and tab is \t.">]
+        Child : ChildRecord
+    }
+
 [<ArgParser true>]
 type ChoicePositionals =
     {

ConsumePlugin/DuArgs.fs

@@ -76,6 +76,55 @@ type WithModeHelpArgs =
         Mode : Mode
     }
 
+/// A union of alternative argument sets may describe itself, for the benefit of every field which
+/// embeds it.
+[<ArgumentHelpText "Which transport to use">]
+type Transport =
+    | Tcp of TcpArgs
+    | Unix of UnixArgs
+
+and TcpArgs =
+    {
+        TcpPort : int
+    }
+
+and UnixArgs =
+    {
+        SocketPath : string
+    }
+
+/// `Fallback` takes the union's own description; `Preferred` overrides it from the field.
+[<ArgParser>]
+type WithTransportArgs =
+    {
+        [<ArgumentPrefix "preferred">]
+        [<ArgumentHelpText "Try this one first">]
+        Preferred : Transport
+        [<ArgumentPrefix "fallback">]
+        Fallback : Transport
+    }
+
+[<ArgumentHelpText "Fetch a URL">]
+type FetchWithHelpArgs =
+    {
+        Url : string
+    }
+
+[<ArgumentHelpText "Push args, not that you'd know it from the case header">]
+type PushWithHelpArgs =
+    {
+        Remote : string
+    }
+
+/// A case's payload record may describe itself, for the benefit of that case; a case is not
+/// reached through a field, so there is no field-level attribute to check first, but the case
+/// itself is a more specific placement than its payload record and so overrides it, exactly as
+/// a field overrides a nested record's own description.
+[<ArgParser>]
+type CommandWithHelp =
+    | FetchCase of FetchWithHelpArgs
+    | [<ArgumentHelpText "Push to a remote">] PushCase of PushWithHelpArgs
+
 /// A union beside a positional sink (default, i.e. Reject-mode): named arguments select the
 /// union case, and every bare token is routed to the sink whichever case wins. An unrecognised
 /// `--key`-shaped token remains fatal. The sink converts, so selection must not depend on the