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/ctx-client-helper.generated.ts

@@ -1,7 +1,7 @@
 // Auto-generated by scripts/gen-ctx-client-helper.mjs — do not edit by hand.
 
 /** Canonical `ctx` API semver from `packages/bare-os-booter/lib/bare-os-ctx-api.js`. */
-export const BARE_OS_CTX_API_CLIENT_VERSION = '1.47.0' as const
+export const BARE_OS_CTX_API_CLIENT_VERSION = '1.48.0' as const
 
 /** Relative path to the hand-maintained TypeScript contract (schema source of truth). */
 export const BARE_OS_CTX_DTS_SOURCE = 'packages/bare-os-booter/lib/bare-os-ctx.d.ts' as const
@@ -10,7 +10,7 @@ export const BARE_OS_CTX_DTS_SOURCE = 'packages/bare-os-booter/lib/bare-os-ctx.d
 export const BARE_OS_CTX_DTS_BAREOS_MEMBER_APPROX = 153 as const
 
 /** Byte length of `bare-os-ctx.d.ts` when this file was generated. */
-export const BARE_OS_CTX_DTS_BYTES = 19312 as const
+export const BARE_OS_CTX_DTS_BYTES = 19405 as const
 
 /** Pointers for external client generators (OpenAPI / JSON Schema follow-ups). */
 export const BARE_OS_CTX_SCHEMA_HINT = {