Let root documents declare ordered positional arguments

[taras]

Why

After #171, a root document can receive declared properties through generated
options, environment variables, aggregate JSON, and the programmatic API.
Simple commands still require a named option for every value:

xmd run hello.md --props-name Ada

Documents should be able to designate an explicit ordered subset of their
properties as positional command-line arguments:

xmd run hello.md Ada

Proposed declaration

arguments: [name]
required: [name]

inputs:
 name:
 type: string
 description: Person to greet
 loud:
 type: boolean
 default: false

arguments supplies order explicitly; property enumeration order is never
used.

Contract

• arguments is root-only CLI metadata, but a Markdown document may retain it
  when imported; it has no effect on component invocation.
• Every entry must name a declared scalar property.
• Duplicate and unknown names fail during inspection.
• Arrays and structured values are not positional. They continue to use
  --props, XMD_PROPS, or programmatic props.
• A positional property no longer receives a generated --props-<name>
  option.
• Its XMD_PROPS_<NAME>, aggregate, and programmatic sources remain available.
• A positional CLI value has the same highest precedence as an individual CLI
  option.
• Positional values appear immediately after the document path and map in the
  declared order.
• Parsing stops at the first option token. Built-in and generated options may
  follow the positionals or precede the document.
• Optional positional arguments follow required positional arguments.
• Missing values may be supplied by lower-priority environment or aggregate
  sources; whole-object validation remains authoritative.
• Extra positional values fail.
• Values beginning with - use aggregate JSON initially; positional --
  escaping is outside this first version.
• Both xmd run document.md ... and the default xmd document.md ... form
  behave identically.

Help

Document-specific help separates positional arguments from named properties:

Arguments declared by hello.md

 name <string>
 Person to greet
 Environment: XMD_PROPS_NAME
 Required

Properties declared by hello.md

 --props-loud[=<boolean>]
 Environment: XMD_PROPS_LOUD
 Default: false

 --props <json>
 Set document properties as a JSON object
 Environment: XMD_PROPS

XMD renders this document-specific presentation from the same binding metadata
used for parsing. Configliere continues to own source resolution, provenance,
and diagnostics; its fixed formatter does not expose the customization needed
for this help contract.

Acceptance

• Required and optional positional values map to the declared property names.
• Scalar decoding follows the same lossless rules as individual named options.
• Environment and aggregate fallbacks retain the documented precedence.
• Positional properties have no duplicate named option.
• Invalid ordering, declarations, missing required values, and extra arguments
  fail before document effects.
• Help shows the ordered arguments without current source values and does not
  execute the document.

Dependency

Implement after #172 and #171. Do not expand #171 with this feature.