Add userland exec path: ELF VFS loader, stack/auxv/TLS setup, and related filesystem/paging fixes

[pradosh-arduino]

Motivation

• Provide a proper userland exec path that can load ELF binaries directly from the VFS and start them in ring 3 with correct arguments, environment and auxiliary vector metadata.
• Ensure ELF program headers can be staged or mapped into user space without requiring a single monolithic file image in memory, to support constrained/streaming VFS backends.
• Improve filesystem robustness (FAT16/FAT32 reads/seeking) and observability for page faults and kernel panics to aid debugging when running user processes.

Description

• Introduced elf_image_info_t and extended ELF APIs with elf_load_from_memory_ex and elf_load_from_vfs_ex to return entry and PHDR metadata, plus helpers for reading program headers from VFS and staging PHDRs into user memory.
• Added VFS-aware ELF helpers (elf_vfs_seek, elf_vfs_read_exact, elf_vfs_read_exact_path) and a streaming mapper elf_map_program_header_from_vfs to map and copy PT_LOAD segments without requiring the whole image buffer.
• Implemented userland process start pipeline: userland_exec, stack/argv/env building including auxv entries (PHDR/PHNUM/PHENT/PAGESZ/ENTRY/RANDOM/PLATFORM/EXECFN), TLS initialization at USER_TLS_VADDR with glibc-compatible TCB layout, and updated enter_userland_at to zero registers before iretq.
• Updated syscall and shell code to support the new exec path: sys_execve now copies argv/envp and calls userland_exec, cmd_exec forwards arguments to userland_exec (with a busybox default), and helpers to copy/free user string arrays were added.
• Filesystem and paging tweaks: added prototype fat32_read_fat, improved FAT16 cluster walking with fat16_next_cluster_with_fallback and robust fat16_read seeking, added fast_virt_to_phys and virt_to_phys alias, and minor paging mapping/flag adjustments to support user mappings.
• Debugging and logging improvements: enhanced page fault logging in isr.c and richer meltdown logging in meltdown.c to include interrupt/error codes and register snapshots.

Testing

• No automated tests were executed for this change (no CI test outputs were provided).

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