Fix reported issues: hint mismatch, broken diagrams, chapter ordering

- async ch15: Fix exercise 4 hint to say Semaphore instead of mpsc
- python ch12: Fix broken Mermaid iterator chain diagram (quote edge labels)
- python ch14: Remove unnecessary escaped parens in Mermaid node label
- c-cpp ch16: Rename sub-chapter file so it sorts after parent in file listings
- c-cpp SUMMARY: Update sub-chapter title to reflect case studies 3-5

async-book/src/ch15-exercises.md

@@ -196,7 +196,7 @@ async fn main() {
 
 Implement an async-aware mutex using channels (without using `tokio::sync::Mutex`).
 
-*Hint*: Use a `tokio::sync::mpsc` channel with capacity 1 as a semaphore.
+*Hint*: Use a `tokio::sync::Semaphore` with 1 permit to serialize access.
 
 <details>
 <summary>🔑 Solution</summary>

c-cpp-book/src/SUMMARY.md

@@ -35,7 +35,7 @@
 - [15. no_std — Rust Without the Standard Library](ch15-no_std-rust-without-the-standard-library.md)
     - [Embedded Deep Dive](ch15-1-embedded-deep-dive.md)
 - [16. Case Studies: Real-World C++ to Rust](ch16-case-studies.md)
-    - [Case Study — Lifetime Borrowing](ch16-1-case-study-lifetime-borrowing.md)
+    - [Case Studies 3–5: Lifetimes, Composability, Trait Objects](ch16-cases-3-5-lifetime-borrowing.md)
 
 ---
 

python-book/src/ch12-closures-and-iterators.md

@@ -173,10 +173,10 @@ flat = [item for sublist in nested for item in sublist]
 
 ```mermaid
 flowchart LR
-    A["Source<br/>[1,2,3,4,5]"] -->|.iter\(\)| B["Iterator"]
-    B -->|.filter\(\|x\| x%2==0\)| C["[2, 4]"]
-    C -->|.map\(\|x\| x*x\)| D["[4, 16]"]
-    D -->|.collect\(\)| E["Vec&lt;i32&gt;<br/>[4, 16]"]
+    A["Source<br/>[1,2,3,4,5]"] -->|".iter()"| B["Iterator"]
+    B -->|".filter(x: x%2==0)"| C["[2, 4]"]
+    C -->|".map(x: x*x)"| D["[4, 16]"]
+    D -->|".collect()"| E["Vec&lt;i32&gt;<br/>[4, 16]"]
     style A fill:#ffeeba
     style E fill:#d4edda
 ```

python-book/src/ch14-unsafe-rust-and-ffi.md

@@ -13,7 +13,7 @@ Python never gives you direct memory access.
 flowchart TB
     subgraph Safe ["Safe Rust (99% of code)"]
         S1["Your application logic"]
-        S2["pub fn safe_api\(&self\) -> Result"]
+        S2["pub fn safe_api(&self) -> Result"]
     end
     subgraph Unsafe ["unsafe block (minimal, audited)"]
         U1["Raw pointer dereference"]