Upgrade the pinned Rust toolchain to 1.98

[hyperb1iss]

rust-toolchain.toml pins 1.95.0 (2026-04-14). Stable is 1.97.1, and
1.98.0-beta is already out, so 1.98 stable is roughly two weeks away.

Nothing is currently blocked by 1.95 — this is hygiene, not a fix.

Go straight to 1.98

Bumping to 1.97.1 now means paying the upgrade cost twice in a month. The
version bump itself is a one-line change; everything expensive rides along with
it, and none of it is worth repeating:

• Clippy churn. The workspace runs clippy::pedantic with -D warnings.
  Every release adds or retunes lints, so the real work is a cleanup sweep
  across the tree. That wants to be its own reviewable diff, not a passenger on
  an unrelated branch.
• Full Servo rebuild. A toolchain change invalidates the whole dependency
  tree, Servo included. Warm the Servo cache (servo-cache-warm.yml) before
  the first CI run on the new pin, or expect the lane to time out.
• Everyone's caches. rust-toolchain.toml is the single source of truth for
  both CI and contributors, so the rebuild lands on every machine at once.

Checklist

• Wait for 1.98.0 stable
• Warm the Servo cache on the new toolchain first
• Bump channel in rust-toolchain.toml
• Sweep new clippy::pedantic findings in a dedicated commit
• Confirm just verify is green on Linux and Windows
• Leave rust-version = "1.94" alone unless the upgrade actually uses a
  newer language or stdlib feature — MSRV is a promise, not a mirror of the
  pin

Note for Windows contributors

scripts/cargo-cache-build.ps1 now asserts the resolved rustc host is
x86_64-pc-windows-msvc before building. If a second Rust is ahead of rustup on
PATH (a GNU-host install is the usual case), the build stops with an
explanatory error instead of failing later inside a dependency's build script
with a mingw linker error or an E0514 rustc mismatch. Worth knowing when the
pin moves, since a stale shadowing toolchain looks exactly like a broken bump.