v0.7.0: Secure Boot chain, boot rules + decision webhook, tokenized answer files

Three features, all zero-toggle and principle-clean (single static musl
binary, container-first, no test certs, no client trust-store changes).

Secure Boot via signed shim+GRUB (automatic):
- The v0.6.1 escalation ladder gains a third rung: Firmware -> Builtin
  -> Shim. Secure-Boot firmware downloads our unsigned iPXE but refuses
  to execute it — indistinguishable from a failed chainload — so after
  two unconfirmed attempts the MAC is offered Fedora's Microsoft-signed
  shimx64.efi, which loads the signed GRUB, which fetches a
  server-rendered grub.cfg. Fully signed chain, SB stays on.
- scripts/fetch-shim.sh pulls shim-x64/grub2-efi-x64 (+aa64 best-effort)
  from the official Fedora 43 packages and ships the EFI binaries
  byte-for-byte unmodified; Dockerfile fetch stage gained rpm2cpio/cpio.
- New grub_script renderer (Linux kernel entries only — signed GRUB only
  boots signed kernels; sanboot/wimboot have no signed equivalent and
  are omitted with an explanatory menu line).
- TFTP server gains a DynamicAsset hook for server-rendered names
  (grub.cfg); HTTP serves the same config under /ipxe/grub.cfg for
  native UEFI HTTP Boot chains. Arch-aware fallback walks back down the
  ladder where no shim exists (BIOS, IA32).

Boot rules + decision webhook (open 'Matrix Boot'):
- Ordered first-match-wins rules over MAC prefix + client arch (the DHCP
  proxy now bakes arch into the boot.ipxe chain URL), generalizing
  per-MAC pins. Persisted to boot_rules.json; GET/PUT /api/boot-rules;
  rules editor + webhook field on the Hosts tab.
- Optional pixiecore-style webhook: unmatched boots GET
  <url>?mac=&arch= and 200 {"target":"id"} chains to it. Fail-open
  with a 2s budget — a dead endpoint can never block PXE.
- Decision order: exact pin -> rules -> webhook -> menu. Empty config
  is byte-for-byte the previous behavior.

Tokenized answer files (the post-WDS/CVE-2026-0386 hardening):
- Every generated unattended URL (inst.ks / preseed url / autoinstall
  seed) now carries a 4h boot-scoped token; /unattended/{id} and the
  cloud-init seed routes require it (or an operator session) once an
  admin exists. Stops answer-file credential harvesting by anything
  else on the network. No toggle; setup-mode installs stay open.

Validation: clippy clean, fmt clean, 290 workspace tests green
(+18 new across boot_tokens, boot_rules, arch ladder, escalation,
grub renderer, and four new full-flow integration tests).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/tftp/src/lib.rs

@@ -14,4 +14,4 @@
 
 pub mod server;
 
-pub use server::TftpServer;
+pub use server::{DynamicAsset, TftpServer};