ci(review): add Copilot code review instructions (#10012)

.github/instructions/c-code.instructions.md

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+---
+applyTo: "src/**/*.c,src/**/*.h"
+---
+
+# LVGL C Source Review Rules
+
+## Memory Safety
+
+- Every `lv_malloc` / `lv_realloc` must have a NULL check immediately after
+- Every allocation must have a matching `lv_free` on all exit paths (including error paths)
+- `lv_free(NULL)` is safe — don't add redundant NULL checks before it
+- Pointer parameters: NULL-check before dereference when caller could reasonably pass NULL
+- Array access must be bounds-checked
+
+## Error Handling
+
+- Functions returning `lv_result_t` must be checked by callers
+- Error paths must clean up all allocated resources before returning
+- Graceful runtime fallback is preferred over assert for recoverable failures (e.g. cache allocation failure should degrade, not crash)
+- Add `LV_LOG_WARN` for unexpected but recoverable conditions
+- Add `LV_LOG_ERROR` for conditions that indicate a bug
+
+## Performance
+
+- No heap allocation in hot paths (draw loops, event handlers, timer callbacks)
+- Prefer stack allocation for small temporary buffers
+- Avoid `lv_obj_is_valid()` in internal callbacks — it walks the entire object tree
+- Cache-friendly access: sequential over random
+- Struct member ordering: pointers first, then int32_t, then smaller types/bitfields — minimizes padding
+
+## Portability
+
+- No platform headers without `#if` guards
+- No compiler extensions without fallback
+- No `float`/`double` in core code unless behind `LV_USE_FLOAT`
+- Use `LV_ATTRIBUTE_*` macros for alignment/section placement
+- Use `"%" LV_PRId32` for `int32_t` format strings, not `"%d"`
+
+## Include Structure
+
+- `.h` files: `#ifndef LV_<MODULE>_H` / `#define` / `#endif`
+- Feature-gated files: keep `#if LV_USE_<FEATURE>` near the top and the matching `#endif` at the bottom; necessary includes (module headers, private headers) may appear before the feature guard
+- Never include test headers in production source files