closes #4 Refactor DBC parsing into TypeFactory statement handlers

README.md

@@ -151,8 +151,11 @@ Files:
 
 - `dbc_parser.h`
 - `dbc_parser.cpp`
+- `dbc_statement_handler.h`
+- `dbc_statement_handler.cpp`
+- `typefactory.h`
 
-These parse the DBC file itself.
+The parser reads files and reports errors; registered statement handlers parse DBC constructs.
 
 ### Demo
 
@@ -767,20 +770,47 @@ That is why `DecodeDatabase` is a separate layer.
 
 ---
 
-## Why No Abstract Factory Is Used
+## Statement Handler Factory
 
-At the current stage, abstract factory is intentionally avoided.
+`DbcParser` owns file reading, trimming, line numbers, and error reporting.
+Each `ParseFile()` call creates a fresh `DbcParseContext` and a
+`DbcStatementDispatcher`. The context contains the parsed database and current
+frame; handlers receive it by reference and retain no parsing state.
 
-The current design is already clean:
+The dispatcher uses `TypeFactory<std::string, DbcStatementHandler>` to create
+registered handlers once per parsing session, then reuses them for all matching
+lines:
 
-```text
-parser → parsed database → decode database
-                      ↘ tree builder → UI tree
+- `BO_` → `DbcFrameHandler`
+- `SG_` → `DbcSignalHandler`
+- `CM_` → `DbcCommentHandler`
+
+Grammar implementations live in `dbc_statement_handler.cpp`; their interface,
+context, and dispatcher live in `dbc_statement_handler.h`. `typefactory.h`
+provides the generic registry and object creation.
+
+To add a construct, implement a default-constructible handler deriving from
+`DbcStatementHandler` with:
+
+```cpp
+void Handle(const std::string &line, DbcParseContext &context) const override;
 ```
 
-Introducing factory layers now would increase complexity without solving an immediate problem.
+Register it in the dispatcher constructor:
 
-If later the project requires multiple output representations or multiple build strategies, that can be added then.
+```cpp
+RegisterType<DbcValueHandler>("VAL_"); // after implementing DbcValueHandler
+```
+
+Registration creates one handler; replacing a registration replaces that instance.
+Register during initialization before dispatching. A line is routed using its
+full first whitespace-delimited keyword, so `BO_TX_BU_` cannot match `BO_`.
+Unregistered keywords remain ignored. Existing `SG_` and `CM_` syntax policies
+are preserved, including requiring a literal space after those keywords.
+
+This factory handles statements inside DBC files. A separate format-level
+factory can be introduced when another file format is implemented. The decoder
+and database builders remain independent of statement dispatch.
 
 ---