restructure source and refactor set_text_body and set_html_body

CLAUDE.md

@@ -0,0 +1,56 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Build Commands
+
+```bash
+cargo build              # Build the library
+cargo build --release    # Build with optimizations (LTO enabled)
+cargo test               # Run all tests
+cargo test test_name     # Run a specific test
+cargo doc --open         # Generate and view documentation
+```
+
+## Architecture
+
+This is a Rust library for parsing, modifying, and serializing email messages (RFC 5322/MIME). It uses copy-on-write semantics to minimize allocations when working with email data.
+
+### Core Types
+
+- **`CowMail<H>`** (`lib.rs`): Main email message struct. Holds headers, body bytes, and nested subparts for multipart messages. Uses `Cow<[u8]>` for zero-copy parsing with owned modifications.
+
+- **`CowMailReader`** (`lib.rs`): Streaming reader implementing `std::io::Read`. Allows efficient serialization without full memory allocation.
+
+- **`CowHeader`** (`header.rs`): Single email header with copy-on-write storage. Tracks byte indices for key/value positions within raw data.
+
+- **`Header` trait** (`header.rs`): Interface for header implementations including parsing, serialization, and RFC 2045 parameter handling.
+
+- **`HeaderMap` trait** (`headers.rs`): Collection operations for headers - find, iterate, add, replace, remove.
+
+- **`Mail` trait** (`lib.rs`): High-level operations for email bodies - content decoding, content-type detection, multipart handling.
+
+- **`CowMailError`** (`lib.rs`): Error type for mail operations (generic errors, parse errors, missing headers).
+
+- **`BodyType`** (`lib.rs`): Enum for body type selection (HTML, PLAIN).
+
+- **`MessageType`** (`lib.rs`): Enum distinguishing SinglePart from MultiPart messages (with boundary).
+
+- **`ContentType`** (`lib.rs`): Wrapper around `ParsedContentType` with conditional defaults for multipart/digest.
+
+### Key Design Patterns
+
+**Copy-on-Write**: Parsed messages borrow from the original byte slice. Only modified fields become owned. The `serialize()` method returns `Vec<Cow<[u8]>>` to preserve this optimization.
+
+**Multipart Handling**: `CowMail` recursively contains `subparts: Vec<CowMail>`. Methods like `make_mixed()`, `make_alternative()`, and `make_related()` transform single-part messages into multipart containers. The original content becomes the first subpart.
+
+**Body Part Navigation**: `get_body_part(BodyType)` traverses the MIME tree to find text/plain or text/html parts, correctly handling `multipart/alternative`, `multipart/related` with `start` parameter, and attachments. The internal `BodyStatus` enum tracks whether a part is a body, not a body, or needs traversal.
+
+### Content Encoding
+
+Body content is automatically encoded/decoded using the `lettre` and `mailparse` crates. `set_content()` automatically selects transfer encoding (quoted-printable for UTF-8 text) and sets appropriate headers.
+
+### High-Level Body Manipulation
+
+- `set_text_body()` / `set_html_body()` / `set_body()`: Intelligently insert or update body parts, creating `multipart/alternative` structures as needed
+- These methods use `get_body_parent_part_mut()` to find the correct insertion point in nested MIME structures