build: update the pinned Rust toolchain to 1.98.0

[sunerpy]

Summary

The pinned Rust toolchain moves from 1.96.0 to 1.98.0, at the owner's request on 2026-10-01.

File	Change
rust-toolchain.toml	channel = "1.98.0"
.github/workflows/ci.yml	every dtolnay/rust-toolchain step (7) → toolchain: 1.98.0
.github/workflows/release.yml	every dtolnay/rust-toolchain step (3) → toolchain: 1.98.0
.github/scaffold.json	RUST_TOOLCHAIN placeholder → 1.98.0
Cargo.toml	rust-version = "1.98"
clippy.toml	msrv = "1.98"

The MSRV moves with the pin, the way #271 set them. Nothing in CI builds an older toolchain, so an MSRV of 1.96 would be an untested claim. As a result, building from source (cargo install --git) now needs Rust 1.98 or newer. The release binaries are unaffected.

Why now: this shell exports RUSTUP_TOOLCHAIN=1.98.0, which overrides rust-toolchain.toml. Local gates therefore ran 1.98 while CI ran 1.96. #291 hit this when CI's 1.96 clippy rejected code that 1.98's clippy accepts. With this PR, local and CI use the same toolchain.

The only remaining 1.96 in the tree is an example inside a comment in .aws/buildspec/windows.yml. It documents a >= 1.85 version check and is not a pin.

Verification

env -u RUSTUP_TOOLCHAIN make pre-ci at cb92dfe, with a clean tree, so rust-toolchain.toml selects the toolchain: rustc 1.98.0, clippy 0.1.98.

• fmt-check, docs-check and workflow-lint pass;
• clippy -D warnings passes with msrv = "1.98";
• 3961 tests passed, 0 failed;
• the release build, the guardrail, the script tests and the archive smoke pass;
• pre-ci exit=0.

The toolchain-only change has no product behavior to test first.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #292   +/-   ##
=======================================
  Coverage   95.31%   95.31%           
=======================================
  Files         161      161           
  Lines       99130    99130           
=======================================
  Hits        94484    94484           
  Misses       4646     4646           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.