fix(python-book): improve mermaid diagrams ordering and readability

python-book/src/ch01-introduction-and-motivation.md

@@ -404,6 +404,7 @@ graph LR
         PY_GIL --> PY_OS["OS / Hardware"]
     end
 
+    PY ~~~ RS
     subgraph RS["🦀 Rust"]
         direction TB
         RS_CODE["Your Code"] --> RS_NONE["No runtime overhead"]

python-book/src/ch04-control-flow.md

@@ -218,6 +218,8 @@ flowchart LR
         P2 --> P4["result used later"]
         P3 --> P4
     end
+
+    Python ~~~ Rust
     subgraph Rust ["Rust — Expressions"]
         R1["let result = if cond"] --> R2["{ 'yes' }"]
         R1 --> R3["{ 'no' }"]

python-book/src/ch05-data-structures-and-collections.md

@@ -195,6 +195,7 @@ flowchart LR
         PH --> PHT["height: float obj"]
         PH --> PD["__dict__"]
     end
+    Python ~~~ Rust
     subgraph Rust ["Rust Struct (Stack)"]
         RW["width: f64<br/>(8 bytes)"] --- RH["height: f64<br/>(8 bytes)"]
     end

python-book/src/ch10-traits-and-generics.md

@@ -322,6 +322,7 @@ flowchart TB
         VT --> I1["Article::summarize()"]
         VT --> I2["Tweet::summarize()"]
     end
+    Static ~~~ Dynamic
     style Static fill:#d4edda
     style Dynamic fill:#fff3cd
 ```

python-book/src/ch11-from-and-into-traits.md

@@ -63,7 +63,7 @@ let n: i32 = "42".parse()?;                    // Propagate error with ?
 ### The From/Into Relationship
 
 ```mermaid
-flowchart LR
+flowchart TB
     A["impl From<A> for B"] -->|"auto-generates"| B["impl Into<B> for A"]
     C["Celsius::from(Fahrenheit(212.0))"] ---|"same as"| D["Fahrenheit(212.0).into()"]
     style A fill:#d4edda

python-book/src/ch15-migration-patterns.md

@@ -251,7 +251,7 @@ let db_host = get_config()["database"]["host"].as_str().unwrap();
 ## Incremental Adoption Strategy
 
 ```mermaid
-flowchart LR
+flowchart TB
     A["1️⃣ Profile Python<br/>(find hotspots)"] --> B["2️⃣ Write Rust Extension<br/>(PyO3 + maturin)"]
     B --> C["3️⃣ Replace Python Call<br/>(same API)"]
     C --> D["4️⃣ Expand Gradually<br/>(more functions)"]