feat: add ext2 write support

[kwrx]

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[Copilot]

Pull request overview

This PR implements write-path functionality for the ext2 filesystem, including inode/block allocation, directory entry mutation, truncate/unlink support, and syscall-level O_TRUNC handling to enable basic POSIX file updates.

Changes:

• Add ext2 write support (block/inode allocation, file writes, truncate, create, unlink) and wire new ops into mount/inode creation.
• Refactor ext2 low-level utilities (block/group desc I/O, inode sizing helpers, directory entry helpers) to support the write path.
• Add a userspace ext2 write/truncate/unlink regression test.

Reviewed changes

Copilot reviewed 23 out of 24 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
libk/utils/cache.c	Fix cache commit to pass the correct cached value to commit handler.
kernel/syscalls/257_openat.c	Add O_TRUNC handling via vfs_truncate() for regular files opened for write.
kernel/fs/ext2/utils/ext2_inode.c	Implement inode allocation/free and block-mapping based inode data read/write/free.
kernel/fs/ext2/utils/ext2_dir.c	Add helpers to add/remove directory entries and map dirent types.
kernel/fs/ext2/utils/ext2_block.c	Add block alloc/free, superblock/group-desc read/write, and harden block I/O with validation + error returns.
kernel/fs/ext2/ext2.h	Add layout asserts, inode size helpers, and expose new ext2 utility APIs + ops.
kernel/fs/ext2/ext2_write.c	Rewrite ext2 write logic to allocate blocks on demand and support holes.
kernel/fs/ext2/ext2_unlink.c	Add ext2 unlink implementation for regular files.
kernel/fs/ext2/ext2_umount.c	Ensure ext2 block cache is destroyed on unmount.
kernel/fs/ext2/ext2_truncate.c	Add ext2 truncate implementation (grow with zero-fill, shrink freeing blocks).
kernel/fs/ext2/ext2_setattr.c	Enforce read-only mounts and commit inode metadata updates via ext2 icache commit.
kernel/fs/ext2/ext2_readlink.c	Update to new inode-data API and add ext2 lock around on-disk readlink path.
kernel/fs/ext2/ext2_readdir.c	Rewrite readdir to robustly parse entries and emit multiple results per call.
kernel/fs/ext2/ext2_read.c	Rewrite read path to new inode-data API with locking and bounds checks.
kernel/fs/ext2/ext2_mount.c	Add feature validation, compute group counts safely, and wire in setattr/creat/unlink ops and caches.
kernel/fs/ext2/ext2_getattr.c	Fix time fields and use unified inode size getter; set blksize from statvfs.
kernel/fs/ext2/ext2_fsync.c	Flush inode cache and forward fsync to underlying device inode.
kernel/fs/ext2/ext2_finddir.c	Add shared inode creation helper and harden directory parsing/search logic.
kernel/fs/ext2/ext2_creat.c	Add ext2 create implementation for regular files (alloc inode + add dir entry).
kernel/fs/ext2/ext2_cache.c	Make inode cache allocate ext2 inode-sized entries and persist on commit when writable.
drivers/dev/block/main.c	Invalidate per-device block cache at start of write path.
apps/test/ext2-write-test/Makefile	Add build rules for a new ext2 write regression test app.
apps/test/ext2-write-test/main.c	Add userspace test covering write patterns, holes, truncate grow/shrink, O_TRUNC, and unlink.
.gitignore	Ignore AGENTS.md and normalize .pdb ignore line formatting.

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