Shift heading depth when expanding an invoked document

[taras]

Story

As the author of a document that invokes another document, I want the invoked
document's headings to fit the outline where its content appears, so neither
author needs to know the other's heading depth.

Example

README.md contains:

## Bootstrap

<BootstrapNpmPackage />

The invoked document is also runnable by itself and begins with:

# Bootstrap a package

When run directly, its title remains # Bootstrap a package. When invoked under
## Bootstrap, the rendered output becomes:

## Bootstrap

### Bootstrap a package

Neither source document changes.

Current problem

Invoked content currently emits the heading levels written in its source. A
standalone document correctly begins with #, but that same heading can appear
beneath a ## section when another document invokes it, making the rendered
outline move backward.

Demoting the invoked source breaks its standalone outline. Promoting the caller
distorts the containing guide. Both make one document depend on where another
document happens to use it.

Rendering rule

Heading adjustment is a property of rendered output, not a source edit.

• A document rendered as the root keeps its authored heading levels.
• When content is emitted beneath a level-N heading, the shallowest heading in
  that content becomes level N+1.
• Every deeper heading shifts by the same amount.
• A document whose shallowest heading is ## composes by the same rule; the
  engine does not assume every document begins at #.
• Nested invocation accumulates the shifts from each containing outline.
• A heading shifted beyond Markdown's sixth level renders as ######. The run
  continues without warning or failure.

The rule applies to every heading in emitted content, whether it came from an
invoked document, a projected string, or output() from an eval block. Authors
do not need separate rules for different output origins.

There is no opt-out. Automatic adjustment is presentation behavior and requires
no syntax in either document.

What does not change

• Target discovery and target addressing continue to use source structure.
• Headings from projected content do not become addressable targets.
• Expansion identity, source ranges, journal records, and replay remain based on
  the original source.
• A root document's output remains unchanged.

The execution records the same operations regardless of where a document is
invoked. Only the text presented to the reader changes.

Implementation boundary

The output layer must know the containing heading depth when it emits projected
content. Today the CLI receives plain string chunks without that structure.

The implementation must carry enough presentation context to adjust headings
without making heading depth part of document identity, target selection, or
the persisted source model.

Acceptance

• A document beginning at #, invoked beneath ##, renders its title at ###
  and shifts the rest of its outline equally.
• The same document run directly still renders its original # title.
• A document whose shallowest heading is not # shifts relative to that
  shallowest level.
• Nested invocation accumulates heading shifts.
• Invoked documents, projected strings, and eval output follow the same rule.
• A heading shifted beyond level six renders at level six and does not fail.
• The target catalog is identical before and after the change.
• Journal and replay identity remain unchanged.

Evidence

Render the same content at the root, one level deep, and two levels deep. Assert
the complete emitted outline and an unchanged target catalog.

Cover invoked-document, string-projection, and eval-output origins. A negative
control that removes heading adjustment must fail the nested cases while the
standalone case still passes.

Related work

• Bootstrap npm packages as a real xmd run: elicit the OTP #276 and ✨ Bootstrap an npm package as a real xmd run #308 introduced the README bootstrap composition where this became
  visible.