Support discriminated unions of alternative argument sets

[Smaug123]

Stacked on #567. The feature this whole stack was building toward.

What you can now write

type FooArgs = { Foo : int }
type BarArgs = { Bar : int ; Baz : int }

[<ArgParser>]
type Args =
    | FooCase of FooArgs
    | BarCase of BarArgs

--foo=3 parses to FooCase { Foo = 3 }; --bar=8 --baz=9 to BarCase { Bar = 8; Baz = 9 }. --foo=3 --bar=8 fails with Arguments select more than one alternative: FooCase (via --foo=3), BarCase (via --bar=8); --bar=8 alone fails with BarCase's own Required argument '--baz' received no value; supplying nothing fails with No arguments were supplied to select one of: FooCase, BarCase. Unions compose: a record field may be union-typed (with other fields alongside), payload records may contain nested records, defaults, flags, etc., and when no argument touches a union, the unique case which needs no arguments is selected as the fallback.

Semantics

Exactly the shape-first, exactly-one-interpretation model from the design discussion: selection depends only on which argument names appear, never on whether values convert, and argument names are globally unique across cases (the existing collision check already enforces this), so parsing stays linear and a malformed value can never flip the selected case. The erased kernel's select was already property-tested against the exhaustive expand-every-alternative reference semantics in #566; this PR wires the generator to it: union lowering into the parse tree, tree-shaped erased schemas, ParseOutcome.Success carrying the selection, and selection-aware assembly (the slots of unselected cases are never read).

Generation-time checks (all verified to fire, documented in ArgParserConflictTests.fs)

• Two cases both satisfiable with no arguments are rejected (an empty command line could not choose).
• Positional args may not appear inside union cases, nor alongside a union arg — ownership of an unrecognised token would become ambiguous.
• The argument name help is reserved in any casing (it always meant help at runtime; generation now refuses it).
• Union cases must hold exactly one field, a record defined alongside the union.

Deferred follow-ups

• Help text for union schemas is the flat list of every case's arguments; grouping by case ("exactly one of:") is a follow-up.
• Sharing an argument name between two cases of one union (with the exponential-fallback parser) is deliberately unsupported.

🤖 Generated with Claude Code

[Smaug123]

(I think at some point we can relax this.)