feat: replace Show with structural If / Else control flow

[taras]

Summary

Replace the spec-only Show conditional-rendering design with first-class structural If and optional Else directives.

<If condition={hasFailures}>
## Test failures

<FailureReport />
<Else>
All checks passed.
</Else>
</If>

Rationale

Show suggests visual visibility. This feature is control flow: it chooses one branch to expand and render, while the other branch must not execute. If makes that behavior explicit.

Scope

• Implement If and Else as structural built-ins in the expansion engine, analogous to Capture; do not resolve them as ordinary filesystem components.
• Replace specification references and examples for Show with If.
• Replace the proposed when/string-fallback API with a required boolean condition prop and optional block-form Else.

Semantics

• condition resolves in the invocation eval environment and must be boolean.
• Else is optional, accepts no props, and may appear once as a direct child of If.
• If true, expand and render only children before Else.
• If false, expand and render only Else children; without Else, render nothing.
• The unselected branch must not import components, run eval/exec blocks, call providers, or create journal entries.

Acceptance criteria

• If condition=true renders its children; condition=false renders no output without Else.
• Expression conditions and lexical eval bindings resolve in the selected branch.
• Only the selected branch expands and performs work; the other branch creates no journal entries.
• Nested If blocks work independently.
• Missing or non-boolean condition produces a clear validation error.
• Else outside If, repeated Else, props on Else, and self-closing Else produce clear errors.
• README and executable-MDX, code-review-agent, and oxlint-sensor specifications use If.
• Deno, Node, and Bun tests cover selection, nesting, validation, and non-execution of unselected branches.

Non-goals

• ElseIf.
• Truthy/falsy coercion.
• Positional component arguments.