feat(booter): POSIX/P2P roadmap — syscalls v4, mirror VFS, seeder snapshot hints

- Bump ctx API to 1.31.0; extend /proc/bare_os/syscalls.json schema 4 (fdModel, signalModel)
- Add /mirror/aux* routing for auxiliary Hyperdrives; optional BLAKE2b-keyed /bin cache (BARE_OS_VFS_BIN_CACHE_BLAKE2B)
- Wire seeder snapshotHintsJson from BARE_OS_SEED_* env; add seeder unit tests
- Shell: trap -p; docs for jobs/bg/trap; sync schemas, examples, compatibility matrix, handbook ch.9
- Coreutils: Issue 7 man option rows for cp/mv/ln/find/grep/sed/awk/tar/test/true; xcu-issue7-sweep tests
- Docs: env appendix, vault-threat-model, node-vs-bare-host-matrix, test:bare, CHANGELOG 1.31.0

kernel/etc/bare-os/syscalls.example.json

@@ -1,6 +1,6 @@
 {
-  "schemaVersion": 3,
-  "ctxApiVersion": "1.30.0",
+  "schemaVersion": 4,
+  "ctxApiVersion": "1.31.0",
   "posixProfile": {
     "id": "bare-os-posix-like",
     "version": "1.0.0"
@@ -14,6 +14,19 @@
     "ENOENT": 2,
     "EACCES": 13
   },
+  "fdModel": {
+    "schema": 1,
+    "note": "Integer FDs index logical VFS/IPC targets in the guest.",
+    "stdio": [0, 1, 2],
+    "register": "ctx.bareOsRegisterLogicalFd",
+    "processTablePath": "/proc/bare_os/process_table.json",
+    "processTableSchema": 3
+  },
+  "signalModel": {
+    "schema": 1,
+    "note": "ctx.bareOsSendSignal delivers to synthetic pids.",
+    "names": ["HUP", "INT", "KILL", "TERM"]
+  },
   "caps": {
     "note": "example"
   },