fix: batch security review hardening

[MerhiOPS]

Summary

• harden sandboxed filesystem operations against symlink-swap escapes by executing CRUD through capability-scoped directory handles
• reserve internal IPC channels, tighten native fast-path rate limiting, and cap/lock down webview response delivery
• harden plugin/runtime boundaries with panic-safe IPC worker cleanup, bounded stderr capture, storage quota/recovery, randomized grant ids, and dialog title sanitization

Commits

• 74a0350 high
• 4fa3773 low
• 211c98f medium

Validation

• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• pnpm --filter voltkit typecheck
• pnpm --filter voltkit lint
• pnpm --filter voltkit test

Notes

• The reported
  emove_dir_all symlink-following claim did not reproduce as described; the destructive escape scenario was not implemented as part of this PR.
• Touched files still above the 300-line soft cap: crates/volt-core/src/fs/mod.rs, crates/volt-core/src/security.rs, crates/volt-runner/src/plugin_manager/process/io.rs, crates/volt-runner/src/plugin_manager/host_api_storage.rs. No file exceeds the 500-line hard cap.