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

kernel/bin/getconf

@@ -140,6 +140,8 @@ const CONF = {
   _SC_AVPHYS_PAGES: '262144',
   _SC_ARG_MAX: '262144',
   _SC_NPROCESSORS_CONF: '4',
+  /** Online processors (static default; live override via `ctx.bareOsGetconfSysconf` + `BARE_OS_NPROC`). */
+  _SC_NPROCESSORS_ONLN: '4',
   _SC_HOST_NAME_MAX: '255',
   _POSIX_HOST_NAME_MAX: '255',
   _SC_MONOTONIC_CLOCK: '1',