llama.cpp was aligning GGUF tensor data from byte 0 of the file, silently corrupting embedded models.

PR #28993 fixes gguf_init_from_file_ptr() reading from the current file position but aligning the data section from offset 0, so a GGUF stored at an unaligned offset inside a larger file (an Android APK asset, say) loaded without error and returned wrong tensor data. No error, wrong numbers. The reader now aligns from where the GGUF actually starts, so embedded models need no new API: fseek then llama_model_load_from_file_ptr(). The PR also turns mmap off with a warning instead of a ggml assert when data isn't at a 32-byte offset, and adds a test-gguf file_offset mode that writes 7 junk bytes ahead of the GGUF and fails without the fix.