chore: raise rust-version floor to 1.88

The 1.85 claim is now false at the dependency level: the lockfile pins
alkcall 0.7.x, and alkcall 0.7.1 raised its MSRV floor to 1.88, so no
1.85 toolchain can resolve this dependency graph regardless of what
Cargo.toml declares. Raising the floor keeps the claim honest and
aligns with the ecosystem floor (noq's 1.88, matching the QUIC path;
iroh sits above at 1.91). alktty's own code is 1.85-clean (verified:
cargo check under 1.85 passes with the pre-bump lockfile) — the raise
is dependency-driven only, so no clippy fixes are needed.

- Cargo.toml: rust-version 1.85 -> 1.88 (version-line bump deferred to
  release time)

Verification:
- 1.88: cargo test --locked (113 default / 156 all-features pass),
  clippy --all-targets -D warnings
- stable 1.94: test --locked, clippy --all-targets -D warnings,
  fmt, doc --no-deps, wasm32-unknown-unknown check + clippy clean

Cargo.toml

@@ -2,7 +2,7 @@
 name = "alktty"
 version = "0.4.0"
 edition = "2021"
-rust-version = "1.85"
+rust-version = "1.88"
 license = "MIT OR Apache-2.0"
 description = "Terminal session protocol: wire format, TtyBackend trait, TtyAdapter, and typed consumer client. Producer/consumer protocol crate on top of alkcall channels."
 repository = "https://git.alk.dev/alkdev/alktty"