Pin MSRV to Rust 1.88 and verify in CI

[iainmcgin]

Closes #43

The workspace had no declared MSRV. The codebase actually uses let-chains (`if let Some(x) = y && condition`) in 20+ places across the published crates, and let-chains were stabilized in Rust 1.88, so 1.88 is the real minimum.

Changes

• `rust-version = "1.88"` added to `[workspace.package]` in the root `Cargo.toml`
• `rust-version.workspace = true` propagated to all 10 member crates
• New `msrv-check` CI job that runs `cargo check --workspace --all-features --all-targets` on Rust 1.88 to keep the floor honest going forward

Verification

• `cargo +1.88 check --workspace --all-features --all-targets` passes locally
• `cargo +1.88 test --workspace --all-features` passes locally (310+ tests, 0 failures)
• Stable clippy + nightly fmt unaffected

Notes

I initially tried 1.85 (matching the buffa workspace), but it fails for two compounding reasons:

1. Let-chains in `connectrpc-codegen/src/codegen.rs:111` and ~20 other call sites in the published crates - stabilized in 1.88
2. Transitive deps via dev-dependencies (`rcgen` → `time`) and bench dependencies (`redis` → `url` → `idna` → `icu_*`, `pprof` → `home`) require 1.86 - 1.88 themselves

Bumping to 1.88 reflects what the code already requires rather than refactoring the let-chains away.