include: one folder per subsystem, and includes that name it

Sixty-five headers sat side by side in include/, so `#include "irq.h"`
said nothing about where irq.h was or what it belonged to, and errno.h,
stat.h and fcntl.h at the top shadowed the system's for any host tool
built with -I./include. They are in folders now -- arch/ core/ fs/
dev/ gfx/ net/ uapi/ (and ldls/ for the Linux driver layer) -- and
every include names the folder, as Linux's do: "core/proc.h".

Done mechanically and checked the hard way: the kernel built from this
tree disassembles identically to the one built from the tree before
it. The full image boots, and userland is the same set of programs.
Paths in docs and comments follow; docs/guides/layout.md has a table
of what is where, and tools/fix-includes.py rewrites old-style
includes from the folders in the tree, for code written before this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

docs/history/kernel.md

@@ -61,7 +61,7 @@ Fixed:
   it's touched, with a clear message.
 - No TLB invalidation existed anywhere except the full `lcr3` reload on
   context switch — `mappages()`/`vm_unmap_range()` never called `invlpg`.
-  Fixed by adding `invlpg()` (`include/x86.h`) to both functions.
+  Fixed by adding `invlpg()` (`include/arch/x86.h`) to both functions.
 - **`MSR_STAR` was misconfigured** (`syscallinit()`, `kernel/arch/x86_64/vm.c`):
   `SYSRETQ` computes the returned `cs`/`ss` as `STAR[63:48]+16`/`+8`, which
   requires the *compat* 32-bit code selector (`SEG_UCODE32`) in that field —
@@ -76,7 +76,7 @@ Fixed:
   length. A single allocation over 8MB (e.g. `realloc()` growth past that
   size) mapped straight into a neighboring live mapping and hit
   `mappages()`'s `panic("remap")`. Replaced with a real bump allocator
-  (`proc->mmap_next`, `include/proc.h`), so every mapping gets its own
+  (`proc->mmap_next`, `include/core/proc.h`), so every mapping gets its own
   disjoint, exactly-sized range regardless of size.
 - **`uva2ka()`** (`kernel/arch/x86_64/vm.c`) returned only the page-aligned
   base address of a user pointer's page, silently dropping the in-page byte
@@ -105,7 +105,7 @@ Bringing in toybox (see "Coreutils" below) surfaced four more, all fixed:
 - `sys_open`/`sys_openat` required `omode == O_RDONLY` *exactly* to open a
   directory, but `opendir()`-style calls legitimately OR in `O_DIRECTORY`/
   `O_CLOEXEC` alongside it. Fixed to check just the access-mode bits
-  (`O_ACCMODE`, added to `include/fcntl.h`).
+  (`O_ACCMODE`, added to `include/uapi/fcntl.h`).
 - `sys_getdents64` read one `struct dirent` (16 bytes) at a time — a full
   `bmap()`+`bread()` per slot. Directories here are hash-indexed (see
   `tree_search()`/`tree_expand()` below), so a directory with a few dozen
@@ -207,7 +207,7 @@ independently re-reads mode fields from the same block.
 Splash/progress bar/boot menu rendering is a small standalone pixel
 renderer in `stage2main.c` (not the kernel's `vga.c` — paging isn't set up
 yet, so the LFB physical address from the VBE block is used as a plain
-pointer) reusing the kernel's own font data (`include/font.h`) directly.
+pointer) reusing the kernel's own font data (`include/gfx/font.h`) directly.
 It fills the framebuffer with PSOGM's purple (matching
 `kernel/core/console.c`'s panic screen) and draws "SCHOS" centered, then:
 
@@ -359,7 +359,7 @@ one tick and returned `-EINTR` unconditionally. mksh's wait loop had no
 way to ever learn the child had exited, hanging forever.
 
 Real signal delivery was implemented from scratch:
-- `struct proc` (`include/proc.h`) gained `sig_handler[32]`/
+- `struct proc` (`include/core/proc.h`) gained `sig_handler[32]`/
   `sig_restorer[32]` (populated by `sys_rt_sigaction()`,
   `kernel/sys/sys_sig.c`), `saved_sigtf` (stashed pre-signal trapframe),
   and `in_signal` (re-entrancy guard). `allocproc()` zeroes these for a
@@ -682,7 +682,7 @@ boot rather than being wiped a few hundred milliseconds in.
 **There were three unrelated designs.** The splash was deep purple with gold
 text, deliberately matching the *panic* screen. The console was cream with
 near-black text. So a healthy boot was dressed as a crash, then abruptly
-became an amber terminal. `include/theme.h` is now the one palette, and the
+became an amber terminal. `include/gfx/theme.h` is now the one palette, and the
 rule is: **normal states are calm and dark; only failure is loud.** PSOGM keeps
 the purple entirely to itself, which is what makes it mean something.