feat(cph): implement nested outline manifest and batch/combined export

ADR-0029 — nested outline manifest, supersedes ADR-0008's flat [[parts]]:
- cph-model: recursive loader over manifest.toml containers / element.toml
  leaves; Lesson.parts (pure elements, DFS order) + Lesson.outline (elements
  interleaved with section headings at their DFS-open position); rejects
  ambiguous/incomplete folders and root-vs-container table misplacement
- cph-diag: new DiagCode::ManifestMalformed for carrier-document structure
  errors (discharges an existing TODO)
- cph-typst: augmented manifest now serializes the outline (element/section
  entries) instead of a flat parts array
- render/lib.typ: render-lesson renders section headings at their depth
- examples/TH-141 migrated to 5 nested section containers + 3 root segments,
  byte-identical element order; smoke-verified via cph check/build + pdftotext

ADR-0030 — batch & combined export, extends ADR-0009/0011:
- cph build with no --target batches every declared target (repeatable
  --target for an explicit subset); any target failure => non-zero exit,
  per-target ledger, independent per-target execution
- cph-model: bundle.toml loader (directory + [info]/[targets.*]/ordered
  lessons with per-lesson target overrides)
- cph-typst: augmented bundle manifest (path-prefixed member outlines),
  Engine::{compile_check_bundle,build_bundle_pdf}
- render/lib.typ: render-bundle assembles member lessons under per-lesson
  headings, depth-shifts their own section headings, resets example/lemma
  counters at each lesson boundary by default
- cph-cli: `cph bundle <path> --target <name>` subcommand, same batching
  contract as `cph build`
- new bundle fixtures/tests (cph-model unit + cph-typst through-template PDF
  compile), smoke-verified via a real 2-lesson merged PDF

Verification: cargo fmt/clippy/test clean across the workspace (68 tests);
real cph check/build/bundle runs against TH-141 and a bundle fixture, PDF
content inspected via pdftotext.

crates/cph-model/tests/fixtures/nested/导言簇/manifest.toml

@@ -0,0 +1,14 @@
+[group]
+title = "导言簇"
+
+[[children]]
+kind = "segment"
+path = "segments/子段一"
+
+[[children]]
+kind = "section"
+path = "嵌套子节"
+
+[[children]]
+kind = "segment"
+path = "segments/子段二"

crates/cph-model/tests/fixtures/nested/导言簇/segments/子段一/element.toml

@@ -0,0 +1 @@
+kind = "segment"

crates/cph-model/tests/fixtures/nested/导言簇/segments/子段一/textbook.typ

@@ -0,0 +1 @@
+子段一。

crates/cph-model/tests/fixtures/nested/导言簇/segments/子段二/element.toml

@@ -0,0 +1 @@
+kind = "segment"

crates/cph-model/tests/fixtures/nested/导言簇/segments/子段二/textbook.typ

@@ -0,0 +1 @@
+子段二。

crates/cph-model/tests/fixtures/nested/导言簇/嵌套子节/lemmas/子引理/element.toml

@@ -0,0 +1 @@
+kind = "lemma"

crates/cph-model/tests/fixtures/nested/导言簇/嵌套子节/lemmas/子引理/stmt.typ

@@ -0,0 +1 @@
+子引理陈述。

crates/cph-model/tests/fixtures/nested/导言簇/嵌套子节/manifest.toml

@@ -0,0 +1,3 @@
+[[children]]
+kind = "lemma"
+path = "lemmas/子引理"