feat: configure focused Cirru previews

[tiye]

Adds a backward-compatible CirruFocusOptions API for custom focus/fold markers, root-prefix preservation, and target/non-target folding limits. Keeps focus_cirru_preview defaults unchanged. Includes unit tests, doctests, README docs, and an executable example. Local cargo test, serde-json tests, and clippy all pass.

[Copilot]

Pull request overview

This PR adds a backward-compatible, configurable API for generating focused/folded Cirru “preview” trees, allowing callers to customize markers, preserve a root signature prefix, and tune fold limits while keeping focus_cirru_preview defaults unchanged.

Changes:

• Introduces CirruFocusOptions and focus_cirru_preview_with_options for configurable focused previews, while preserving existing focus_cirru_preview behavior.
• Updates crate exports to expose the new API from the crate root.
• Adds documentation and examples (README + runnable example) and extends unit tests for the new options.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/parser_test.rs	Minor cleanup to use map(Cirru::List) in serde-json parser test.
src/parser.rs	Re-exports CirruFocusOptions and focus_cirru_preview_with_options from the crate root.
src/focus.rs	Implements configurable focus/fold options, root-prefix preservation, and adds tests/doctests.
README.md	Documents the focused preview API and how to customize it.
examples/focus_cirru_preview.rs	Demonstrates both default focusing and customized focusing via options.

Comments suppressed due to low confidence (1)

src/focus.rs:80

• The doc comment for folded_place still hard-codes 'FOLDED:... and claims the placeholder leaf will always print bare. Now that options.folded_marker is user-configurable, the placeholder may not be 'FOLDED:..., and if the marker (or detail) contains spaces/non-ASCII it will be quoted/escaped by the writer (see README escaping example). Updating this comment will prevent misleading API docs.

/// Build a `'FOLDED:...` placeholder: a single bare Cirru symbol leaf.
///
/// Uses the `'` symbol prefix (not `|` string prefix): this is an
/// identifier-like marker, not free-form string data, so `'FOLDED:...`
/// matches Cirru's own convention for such tokens. Deliberately a single

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