feat(sched): Weave tiered scheduler with real kernel-thread context switching

Replace the state-relabel-only round-robin with the SUB-OS "Weave"
scheduler backed by a genuine callee-saved context switch. Threads now
actually run on their own kernel stacks instead of merely having their
state field rewritten.

What was implemented:
- sub_ctx_switch(): per-arch assembly (arch/x86_64/cpu/switch.asm,
  arch/aarch64/cpu/switch.S, arch/armv8i/cpu/switch.S) that saves/restores
  the callee-saved register set (+ flags on x86_64) on each thread's own
  stack, plus a first-run trampoline (sub_thread_trampoline) that launches
  a freshly forged thread frame with entry/arg in callee-saved registers.
- task_forge_frame(): builds the initial saved-context frame per arch so
  the very first switch into a new thread lands on the trampoline; task_t
  gains a pointer-sized ctx_sp so the same struct works on 64- and 32-bit.
- Weave policy (kernel/sched.c): WEAVE_NLANES priority lanes, always
  dispatch the lowest non-empty lane round-robin; a task that yields
  voluntarily is woven UP a lane (interactive), one preempted after
  burning its quantum is woven DOWN (CPU-bound). Cooperative switching
  (sched_yield / task_exit) is always live; timer-driven preemption is
  gated behind sched_set_preempt() and left disarmed for now.
- task_exit() now switches to the next runnable thread via
  sched_exit_current() instead of spinning/halting.
- Boot-time weave_selftest() (x86_64) and sched_dump() diagnostics.

How it was verified:
- Builds clean for all three targets: x86_64 (CROSS_COMPILE=x86_64-linux-gnu-,
  RUSTC="rustc --target x86_64-unknown-none"), aarch64, armv8i. No new
  warnings; the new x86_64 asm carries a .note.GNU-stack section.
- Booted x86_64 under QEMU (TCG): the self-test runs three cooperative
  kernel threads to completion on separate stacks with the expected
  1->2->3 round-robin interleave, reports "20 real context switches,
  3/3 workers exited", returns cleanly to the boot thread, and boot
  proceeds normally to ring-3 init and the shell. No fault/panic.

Left unverified:
- The aarch64/armv8i sub_ctx_switch frames build and link but are not yet
  boot-tested on their QEMU targets (tracked in docs/AUTODEV_ROADMAP.md).
- Timer preemption path is implemented but disarmed; exited kernel threads
  currently leak their stack (reaper is next in the backlog).

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Jk6K69yGS7zYgvoWd6nJxQ

docs/AUTODEV_ROADMAP.md

@@ -0,0 +1,67 @@
+# SUB-OS Autonomous Development Roadmap
+
+This file is the cross-run memory for the continuous-development effort. Each
+run: read it, pick the single highest-value NEXT item achievable in one
+session, implement + verify it, then move it to DONE with a one-line result and
+refine the backlog.
+
+## Known-good build invocations (this host)
+
+- **Toolchain setup:** `apt-get install -y nasm gcc-aarch64-linux-gnu
+  binutils-aarch64-linux-gnu gcc-arm-linux-gnueabihf binutils-arm-linux-gnueabihf
+  qemu-system-x86 qemu-system-arm`; for Rust: `rustup target add x86_64-unknown-none`.
+- **x86_64:** `make x86_64_defconfig && make CROSS_COMPILE=x86_64-linux-gnu-
+  CXX=x86_64-linux-gnu-g++ RUSTC="rustc --target x86_64-unknown-none"`
+- **aarch64:** `make aarch64_defconfig && make`
+- **armv8i:** `make armv8i_defconfig && make`
+- **Boot (x86_64, headless):** `qemu-system-x86_64 -drive format=raw,file=SUB-OS.img
+  -m 256M -netdev user,id=net0 -device e1000,netdev=net0 -serial stdio -display none`
+  (slow under TCG; the shell waits on input, so a timeout at the prompt is normal).
+
+## DONE
+
+- **Weave scheduler + real kernel-thread context switching** (2026-08-25):
+  Replaced the state-relabel-only round-robin with the SUB-OS "Weave" tiered
+  scheduler backed by a genuine callee-saved context switch (`sub_ctx_switch`,
+  per-arch assembly for x86_64/aarch64/armv8i) and a first-run trampoline.
+  Threads run on their own kernel stacks; a task's lane floats on behaviour
+  (voluntary yield → woven up, quantum-burn → woven down). Verified on x86_64
+  under QEMU: a boot-time self-test runs 3 cooperative kernel threads to
+  completion (20 real context switches, correct 1→2→3 round-robin interleave,
+  clean return to the boot thread, then boot proceeds to ring-3 init + shell).
+  ARM switch asm builds on both targets but is not yet boot-tested.
+
+## NEXT (prioritized backlog)
+
+1. **Preemptive Weave scheduling from the timer tick.** The plumbing exists
+   (`sched_set_preempt`, `sched_tick` charges the quantum). Arm it *after* boot
+   completes, make the PIT ISR path re-entrancy-safe (switch on IRQ return, not
+   mid-handler), and add a preemption self-test showing a CPU-bound thread being
+   woven down while an interactive one stays in lane 0. Needs an IRQ-context
+   switch (save the full trap frame, not just callee-saved state).
+2. **Thread reaping / zombie cleanup.** Exited kernel threads currently leak
+   their 16 KB stack + task_t. Add a reaper (per-CPU dead list drained by the
+   idle thread) so `task_exit` hands the corpse to it instead of leaking.
+3. **Boot-test the ARM context switch.** Wire a `weave_selftest()` call into the
+   aarch64/armv8i boot paths and boot under `qemu-system-aarch64`/`-arm` to
+   confirm the AArch64/AArch32 `sub_ctx_switch` frames are correct.
+4. **fork/exec/wait for the ring-3 userland.** Build on the existing ELF64
+   loader + per-process page tables so `sh` can spawn and reap real children,
+   not just the single builtin init.
+5. **An original capability-based IPC channel** ("sub-ports"): typed,
+   capability-addressed message endpoints distinct from the SysV msg/pipe/shm
+   set already present in `ipc/` — leaning into the "object-first" identity.
+6. **SMP bring-up (x86_64):** AP boot trampoline, per-CPU run lanes, and making
+   the Weave lanes per-CPU with work-stealing between them.
+7. **GUI (feat/gui-desktop):** compositor damage tracking, a VFS-backed file
+   manager, a text editor, and a settings panel over the toolkit widgets.
+
+## Notes / gotchas
+
+- `.S` files are run through cpp: no nested `/* ... */` comments.
+- `printk`/vsprintf supports only `c s d i u o x X p %` — no length modifiers
+  (`l`, `ll`) and no field width; cast to `int`/`unsigned`/`void*`.
+- `list_del()` NULLs both links, so `list_empty()` is not a reliable "is this
+  node queued?" test after a delete — check the links directly.
+- During `subos_modular_core_boot()` step 13, hardware interrupts are still
+  masked (enabled at step 14), so cooperative in-boot tests are deterministic.