I1: SML parser — inverse of to_sml()

Adds parse_sml(text) -> Item / try_parse_sml(text) -> optional<Item>
in secs2/sml.hpp.  Round-trips with the existing to_sml() emitter for
every Item shape the codec produces: lists with nesting, ASCII / JIS8,
Binary (decimal and 0xHH literals), Boolean (T/F or 1/0, scalar and
multi-element), U1-U8 / I1-I8 / F4 / F8 vectors, and the optional
`[n]` count syntax (accepted but not enforced).

The parser is whitespace-insensitive outside quoted strings and uses
a small Cursor type for read_word / read_quoted / skip_ws.  Numeric
literals go through strtoul/strtoll/strtod so SML can carry hex,
octal, and decimal interchangeably (the emitter writes hex for Binary
and decimal everywhere else).

11 test cases cover the full round-trip surface, the whitespace
invariant, unknown-tag rejection, the try_parse error-swallowing
variant, and the optional `[n]` count.

secsgem-py has secs/sml.py for the same purpose; this brings the C++
port to parity on the tooling side.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CMakeLists.txt

@@ -43,6 +43,7 @@ add_custom_target(generate_messages DEPENDS ${MESSAGES_HPP})
 
 add_library(secsgem
   src/secs2/codec.cpp
+  src/secs2/sml.cpp
   src/hsms/header.cpp
   src/hsms/connection.cpp
   src/secsi/header.cpp
@@ -110,6 +111,7 @@ add_executable(secsgem_tests
   tests/test_ept.cpp
   tests/test_cem_objects.cpp
   tests/test_modules.cpp
+  tests/test_sml.cpp
 )
 target_link_libraries(secsgem_tests PRIVATE secsgem doctest::doctest)
 target_compile_definitions(secsgem_tests PRIVATE