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};

crates/tftp/src/server.rs

@@ -32,11 +32,19 @@ const ERR_NOT_DEFINED: u16 = 0;
 const ERR_FILE_NOT_FOUND: u16 = 1;
 const ERR_ILLEGAL_OP: u16 = 4;
 
+/// Server-rendered TFTP content for names that aren't embedded assets —
+/// e.g. `grub.cfg` for the signed shim+GRUB Secure Boot chain (v0.7.0),
+/// which is generated from the live boot-entry list per fetch. Kept as a
+/// closure so this crate stays decoupled from the ISO store; the binary
+/// wires it up in `main`.
+pub type DynamicAsset = Arc<dyn Fn(&str) -> Option<Vec<u8>> + Send + Sync>;
+
 pub struct TftpServer {
     bind: IpAddr,
     port: u16,
     clients: Arc<ClientRegistry>,
     metrics: openpxe_core::Metrics,
+    dynamic: Option<DynamicAsset>,
 }
 
 impl TftpServer {
@@ -45,12 +53,14 @@ impl TftpServer {
         port: u16,
         clients: Arc<ClientRegistry>,
         metrics: openpxe_core::Metrics,
+        dynamic: Option<DynamicAsset>,
     ) -> Self {
         Self {
             bind,
             port,
             clients,
             metrics,
+            dynamic,
         }
     }
 
@@ -72,8 +82,11 @@ impl TftpServer {
             let clients = clients.clone();
             let metrics = metrics.clone();
             let bind_ip = self.bind;
+            let dynamic = self.dynamic.clone();
             tokio::spawn(async move {
-                if let Err(e) = handle_rrq(data, from, bind_ip, clients, metrics.clone()).await {
+                if let Err(e) =
+                    handle_rrq(data, from, bind_ip, clients, metrics.clone(), dynamic).await
+                {
                     metrics.record_tftp_err();
                     tracing::warn!(target: "openpxe::tftp", peer=%from, "handler error: {e}");
                 }
@@ -88,6 +101,7 @@ async fn handle_rrq(
     bind_ip: IpAddr,
     clients: Arc<ClientRegistry>,
     metrics: openpxe_core::Metrics,
+    dynamic: Option<DynamicAsset>,
 ) -> anyhow::Result<()> {
     let Some(req) = parse_rrq(&packet) else {
         // Not a well-formed RRQ. A WRQ deserves an explicit refusal —
@@ -117,7 +131,15 @@ async fn handle_rrq(
         return Ok(());
     }
 
-    let Some(file_bytes) = asset_slice(&filename) else {
+    // Embedded assets first; otherwise the dynamic renderer (server-
+    // generated content like the Secure Boot chain's grub.cfg, v0.7.0).
+    let resolved = asset_slice(&filename).or_else(|| {
+        dynamic
+            .as_ref()
+            .and_then(|f| f(&filename))
+            .map(std::borrow::Cow::Owned)
+    });
+    let Some(file_bytes) = resolved else {
         let _ = send_error(&sock, peer, ERR_FILE_NOT_FOUND, "no such file").await;
         tracing::info!(target: "openpxe::tftp", peer=%peer, file=%filename, "404");
         clients.record(