chore(booter): complete P2P/POSIX roadmap — ctx 1.48, VFS, shell, docs

- Bump bareOsCtxApiVersion to 1.48.0; sync CHANGELOG, compatibility matrix,
  syscalls.example.json, ctx d.ts, generated ctx-client helper
- POSIX: getconf _SC_NPROCESSORS_ONLN; shell set -o pipefail + BARE_OS_PIPESTATUS;
  posix_utilities schema v2 + JSON Schema; generated dashboard refresh
- Host/subprocess: bare-subprocess then Node child_process spawn; optional
  backend on ctx.bareOsTrySpawnHostSubprocess; bin-worker WASM wall budget
- VFS: BARE_OS_VFS_SYSTEM_IMAGE_WRITE for system image writes + warm-cache
  eviction path; guest /.bare/account EACCES test; environ TOKEN redaction test
- Docs: corestore snapshot non-goal in package-bare-os-booter; PEAR-RUN links
  → docs/PEAR-RUN.md; POSIX pretest matrix in scripts/README + dev guide;
  environment appendix (PIPESTATUS, WASM_MS, system image write)
- Seeder: keep kernel/ mirror in sync after bundle + coreutils builds

Verified: npm test -w bare-os-booter, npm run pretest

docs/reference/feature-roadmap.md

@@ -799,7 +799,7 @@ Second batch of the same governed program. Traceability: [developer-guide/kernel
 | 88 | BARE_OS_DELEGATE_TRACE unified log format env | done |
 | 89 | Machine-readable capability map JSON proc | done |
 | 90 | Developer-guide cookbook pointer | done |
-| 91 | kernel-program-benchmark-harness boot timings | done |
+| 91 | kernel-program-benchmark-harness boot timings (`kernel_crypto_synthetic_benchmark` suite) | done |
 | 92 | VFS microbench script stub | done |
 | 93 | IPC ping-pong booter test augmentation | done |
 | 94 | Network mock bench DNS/TCP policy path | done |