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-check/tests/pipeline.rs

@@ -59,7 +59,7 @@ name = "broken"
 [info]
 title = "broken"
 
-[[parts]]
+[[children]]
 kind = "frob"
 path = "elements/widget"
 "#,
@@ -123,7 +123,7 @@ name = "broken"
 [info]
 title = "broken"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/does-not-exist"
 "#,
@@ -150,7 +150,7 @@ name = "cov"
 [info]
 title = "cov"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/intro"
 
@@ -276,7 +276,7 @@ name = "sh"
 [info]
 title = "sh"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/intro"
 
@@ -350,7 +350,7 @@ name = "sh"
 [info]
 title = "sh"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/missing"
 
@@ -390,7 +390,7 @@ fn write_markdown_assemble_target_lesson(tmp: &Path, slides: &[(&str, &str)]) {
             parts.push('\n');
         }
         parts.push_str(&format!(
-            "[[parts]]\nkind = \"segment\"\npath = \"segments/{name}\"\n"
+            "[[children]]\nkind = \"segment\"\npath = \"segments/{name}\"\n"
         ));
     }
     std::fs::write(
@@ -468,8 +468,8 @@ fn run_markdown_assemble_target_skips_parts_without_the_field() {
     // Only the first segment has a slides.md; the second is skipped (optional).
     let tmp = tempdir();
     let mut parts = String::new();
-    parts.push_str("[[parts]]\nkind = \"segment\"\npath = \"segments/a\"\n\n");
-    parts.push_str("[[parts]]\nkind = \"segment\"\npath = \"segments/b\"\n");
+    parts.push_str("[[children]]\nkind = \"segment\"\npath = \"segments/a\"\n\n");
+    parts.push_str("[[children]]\nkind = \"segment\"\npath = \"segments/b\"\n");
     std::fs::write(
         tmp.join("manifest.toml"),
         format!(
@@ -527,7 +527,7 @@ name = "md"
 [info]
 title = "md"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/a"
 
@@ -584,7 +584,7 @@ name = "md"
 [info]
 title = "md"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/a"
 
@@ -630,7 +630,7 @@ name = "md"
 [info]
 title = "md"
 
-[[parts]]
+[[children]]
 kind = "segment"
 path = "segments/missing"