chore: bump Rust PPA toolchain to 1.96 (#270)

## Summary

Raises the Rust floor from 1.95 to 1.96 now that the lablup `rustc-release` dependency PPA (`~lablup/+archive/ubuntu/rustc-release`) provides `rustc-1.96`. This keeps the crate MSRV and the Launchpad build toolchain in sync, matching the coordinated bump previously done when the floor moved to 1.95.

## Changes

- **Cargo.toml**: `rust-version` → `1.96` (MSRV floor).
- **.github/workflows/launchpad_ppa.yml**: `vendor_rust` → `1.96.0` and the toolchain description strings for all three distro matrix entries (jammy/noble/resolute).
- **debian/control**, **debian/control.source**: `Build-Depends` → `rustc-1.96 | rustc (>= 1.96)`, `cargo-1.96 | cargo (>= 1.96)`.
- **debian/rules**, **debian/rules.source**, **debian/rules.launchpad**, **debian/rules.launchpad-simple**: toolchain detection now prefers `rustc-1.96`/`cargo-1.96`, and the `dpkg --compare-versions` guards require `>= 1.96`.
- **debian/README.packaging**: floor description, PPA `provides` notes, and troubleshooting guidance updated to 1.96.

## Notes

- `Cargo.lock` needs no change; the lockfile format is unchanged and 1.96 parses the existing v4 lockfile. The workflow's `cargo metadata --locked` validation still holds.
- `debian/README_PPA.md` still references the obsolete `rust-1.85-all` approach; it was already stale relative to the versioned-package build path and is left out of scope for this bump.

## Testing

No Rust source changed (packaging/CI/metadata only). Build verification runs on CI runners with the correct toolchain; the local machine has Rust 1.95.0 installed, which the new floor intentionally rejects.