[core] Use constexpr for PROGMEM arrays

Change progmem_array codegen from `static const` to `static constexpr`.
This ensures the compiler evaluates the array at compile time with no
runtime initialization, and is the correct qualifier for constant data
known at compile time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit_tests/test_cpp_generator.py

@@ -325,7 +325,7 @@ class TestStatements:
             ),
             (
                 cg.ProgmemAssignmentExpression(ct.uint16, "foo", "bar"),
-                'static const uint16_t foo[] PROGMEM = "bar"',
+                'static constexpr uint16_t foo[] PROGMEM = "bar"',
             ),
         ),
     )