feat(model): author 是有序列表;Info(canonical) vs RawInfo(授权 surface) 入契约

一节课可多人署名(教研组),故 canonical author 是**有序列表**而非单值。on-disk
形式接受裸字符串(单作者)或数组(多作者),但此"字符串或数组"二态**只活在加载边界**:
RawInfo 经归一化折叠成 canonical Info,其后系统只见 List。

Lean 母本(ADR-0008 范围):新增 Model/Info.lean,把 Info(canonical,authors: List
String)与 RawInfo/RawAuthor(授权便利 surface)+ 归一化 RawInfo.toInfo 钉死——
canonical 接收端恒为列表,raw 形式不泄漏。ADR-0008 [info] 块补 author-as-list 决策。

impl 对齐:cph-model Info.author: Option<String> → authors: Vec<String>;RawAuthor
untagged enum(One|Many)+ into_vec 归一化;augmented manifest 发射 author 数组
(typst document(author:) 与模板已接受 string/array)。doc 两端互引 spec。

测试:load.rs 单作者→单元素列表;target-configs fixture 加数组作者断言多作者。
验证:lake build 绿(25 jobs);cargo test 全绿;clippy 静默;TH-141(单字符串作者)
check+build PDF 无回归。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/adr/0008-engineering-file-layout-conventions.md

@@ -45,7 +45,7 @@ name = "…"            # folder/display name
 
 [info]                # passed through to render targets verbatim
 title  = "…"
-author = "…"
+author = "…"           # one author; or a list: author = ["…", "…"]
 
 [[parts]]             # ORDER OF THIS ARRAY IS THE LESSON ORDER (ADR-0005)
 kind = "segment"      # one of the known kinds (ADR-0006 / stdlib set)
@@ -63,6 +63,18 @@ that imports elements in order is a *generated build artifact* derived from the
 manifest (see ADR-0007's note that the on-disk encoding of `Lesson` is separate
 from "it's a tree"), never the canonical order.
 
+### `[info].author` is a list; the on-disk single-string form is sugar
+
+A lesson can be authored by several people (a teaching group), so the canonical
+`author` is an **ordered list**, not a single value. For authoring convenience the
+on-disk form accepts **either** a bare string (`author = "…"`, one author) **or** an
+array (`author = ["…", "…"]`). That "string-or-array" union lives **only at the load
+boundary**: the loader normalizes a bare string to a one-element list, and
+everything downstream sees a list. Absent `author` ⇒ empty list. This raw-vs-
+canonical split is pinned in the Lean master as `RawInfo` (the authoring surface)
+vs `Info` (the canonical model whose `authors` is always a list) — see
+`spec/Spec/Courseware/Model/Info.lean`.
+
 ### An element folder is self-describing via `element.toml`
 
 Each element folder carries an **`element.toml`** declaring its kind and its