v0.4 release candidate 1

First snapshot since v0.3.0. Ships the v0.4 platform-hardening cycle that
landed via PR #6 plus the post-merge fs name bug fix.

Highlights:

• ELF-aware bootloader; kernel relocated to 0x100000 with a Multiboot-1
  header so GRUB can boot the same kernel.elf.
• Preemptive scheduler, anonymous pipes, signals, per-process VFS.
• TCP stack with RFC 793 state machine, MSS, window scale, SACK, fast
  retransmit, persist probe, keepalive, TIME_WAIT reap.
• DHCP retry + secondary DNS; 8-entry DNS cache; ARP with TTL eviction.
• SB16 mixer with Q16.16 resampling; full Doom port (Freedoom1/2 + FreeDM).
• Package manager (stink-pkg) with dep resolution and SHA-256 manifests.
• 62+ host-side unit tests gated by CI.
• Kernel text -39% via switch to -Os.

Tagged as -rc1 because the multi-process Doom-while-shell demo (TODO 1)
has not landed yet -- v0.4 final ships when that demo works.