kernel: stat/fsstat/isatty syscalls, and O_APPEND

Four things userland had no way to ask for:

- SYS_STAT: type, link count, size and inode for a path. The trap needs HL
  for its dispatch table, so a call taking both a path and an output buffer
  passes a request block - the shape SYS_NET already established. usr/stat.h
  wraps it, header-only like sock.h, because the block must live in the
  caller's RAM.
- SYS_FSSTAT: blocks/inodes, total and free. The bitmaps are already cached
  write-through in fs_bmbuf, so this is the popcount boot_fs already prints.
- SYS_ISATTY: is this process's stdin the console? The shell needs it to tell
  an interactive session from a script.
- O_APPEND: open a file positioned at EOF. There is no seek syscall, so this
  is the only way to add to a file - without it nothing running ON the Game
  Boy can build a multi-line file, shell scripts included.

Together these back ls -l, df, and `>>`.

1 files changed, 13 insertions, 0 deletions

diff --git a/docs/internals.md b/docs/internals.md
index 3e27bd2..f9c5899 100644
--- a/docs/internals.md
+++ b/docs/internals.md
@@ -234,6 +234,13 @@ blocks 4..  data blocks
   the damage once something else touched the neighbouring inode.
 - **One-block data cache.** Two files open at once thrash it: `cp` of a 3 KiB
   file alternates read/write blocks per byte and takes ~25s. Correct, slow.
+- **Open modes** are read / write / append. There is no seek syscall, so
+  `O_APPEND` (open at EOF) is the only way to add to a file - which is what
+  makes `>>`, and therefore building a multi-line script on the device,
+  possible at all.
+- **`sys_stat` / `sys_fsstat`** back `ls -l` and `df`. The syscall trap needs
+  HL for its dispatch table, so a call taking both a path and an output
+  buffer passes a request block, the shape SYS_NET already uses.
 - **Paths + per-process cwd.** `open`/`mkdir`/`chdir`/`remove`/`ls` take paths
   (absolute `/a/b` or relative `a/b`); each process has a cwd inode (`PROC_CWD`,
   inherited on fork). Shell builtins: `cd`, and a cwd-aware prompt.
@@ -271,6 +278,12 @@ gbos programs are written in **C**, compiled with **SDCC** (the `sm83` port):
 Arguments: the shell leaves `"cmd\0args\0"` at `$A000`; the child inherits it
 through `fork`, `getargs()` returns the raw string, `argv_parse()` tokenizes.
 
+**Scripts.** `sh` is the same program interactively and as a script runner: it
+asks `isatty()` (SYS_ISATTY: is stdin the console?) and, when stdin is a file,
+drops the prompt and the echo. EOF ends that shell - only pid 1 on the console
+treats EOF as "power the machine off", which is why a script used to shut the
+Game Boy down when it reached the end. init runs `/rc` at boot if it exists.
+
 **Toolchain gotchas found the hard way:** SDCC's `--asm=rgbds` mode mis-orders
 instructions (emits `ld [hl],a` before the `ld hl,sp+0` that sets the pointer)
 — so we use the native asxxxx path and INCBIN the blob instead. String