chore(workspace): pin rust-toolchain to stable, set MSRV 1.95

Add rust-toolchain.toml so all developers and CI use the same Rust
channel (latest stable = 1.95.0 as of 2026-04-14). Set rust-version
= "1.95" in workspace Cargo.toml to declare the minimum supported
Rust version explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -14,6 +14,7 @@ resolver = "2"
 edition = "2024"
 version = "0.1.0"
 license = "MIT"
+rust-version = "1.95"
 
 [workspace.dependencies]
 serde       = { version = "1", features = ["derive"] }

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "stable"