feat(c4): add @c4 code level (struct/enum/trait/fn) + code diagram

[nightscape]

What

Adds the bottom of the C4 model. Item-level @c4 code doc markers on struct / enum / trait / fn become curated code-level nodes under their component. The rust adapter previously parsed only module //! docs.

• archidoc-types: CodeElement on ModuleDoc + ir::CodeElement on DirNode (both serde-default, so existing IR JSON stays valid).
• archidoc-rust/parser: extract_code_elements() uses syn to find items whose doc contains @c4 code; captures kind, description, and @c4 uses relationships. Unmarked items are ignored — the diagram is a curated set of load-bearing types, not a dump of every symbol.
• archidoc-rust/walker: aggregates a module's code elements from its directory's source files onto the component's ModuleDoc.
• archidoc-engine: ir_builder propagates code_elements; plantuml::generate_code() renders c4-code.puml (one Component(...) per element, kind as the tech tag, intra-component @c4 uses resolved to qualified ids). Emitted only when a code element exists.

Example

/// @c4 code
/// The seam every storage backend implements.
/// @c4 uses StorageEntity "yields rows as" "Rust"
pub trait FileFormatAdapter {}

/// @c4 code
pub struct StorageEntity {}

pub struct InternalHelper {} // no marker → excluded

→ c4-code.puml with FileFormatAdapter + StorageEntity as components and an arrow between them; InternalHelper is omitted.

Notes

@c4 code items are collected per component directory (the walker aggregates a module's source files). Items in deeper un-annotated subdirectories are not yet picked up — a reasonable follow-up. Test added covering curation, typing, and relationships.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for curated code-level (@c4 code) elements to the architecture IR and emits a dedicated PlantUML “code diagram” for them.

Changes:

• Introduces CodeElement in module docs and IR; overlays extracted elements into the directory tree and merge logic.
• Extends the Rust adapter to parse @c4 code elements (struct/enum/trait/fn) from item docs, including @c4 uses relationships.
• Adds generate_code to emit c4-code.puml and wires it into the CLI render flow.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
core/archidoc-types/src/module_doc.rs	Adds module-doc CodeElement and exposes it on ModuleDoc.
core/archidoc-types/src/lib.rs	Re-exports CodeElement at the crate root.
core/archidoc-types/src/ir.rs	Adds IR-level code_elements on DirNode and defines an IR CodeElement.
core/archidoc-engine/src/tree.rs	Initializes code_elements when building the dir tree.
core/archidoc-engine/src/plantuml.rs	Implements generate_code to emit c4-code.puml.
core/archidoc-engine/src/merge.rs	Merges code_elements alongside other dir fields.
core/archidoc-engine/src/ir_builder.rs	Overlays ModuleDoc.code_elements into IR DirNode.code_elements.
core/archidoc-cli/src/main.rs	Calls generate_code during PlantUML rendering.
adapters/archidoc-rust/src/walker.rs	Extracts code elements across sources (special-casing entry modules).
adapters/archidoc-rust/src/parser.rs	Parses @c4 code elements using syn and adds unit test coverage.
adapters/archidoc-rust/src/cargo_modules.rs	Updates test fixtures to include code_elements.

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