Upgrade to Rust 1.98.1 and align dependencies with EdgeZero #1124

The workspace is pinned to Rust 1.95.0 while EdgeZero moves to 1.98.1 in stackpop/edgezero#366. That PR also takes a set of dependency majors we share with EdgeZero, so the two workspaces have to move together: pinning EdgeZero at a revision that requires fastly 0.13 while this workspace asks for 0.12 puts two incompatible copies of the same SDK in one graph.

Scope

Toolchain — match .tool-versions and rust-toolchain.toml to EdgeZero:

EdgeZero — currently pinned at tag v0.0.7. The latest release, v0.0.8, still carries the 1.95 toolchain and the old majors, so a tag will not do until #366 merges and is tagged.

Shared dependency majors — fastly and log-fastly 0.12 to 0.13, spin-sdk ~6.0 to ~7.0, validator 0.20 to 0.21, sha2 0.10 to 0.11. brotli stays at 8 for the reason #366 measures: async-compression still resolves 8, so taking 9 adds a second decompressor stack rather than replacing one.

Expected fallout

• sha2 0.11 moves RustCrypto to hybrid_array, which drags hmac and chacha20poly1305 with it and changes two API shapes (new_from_slice moves traits; Array loses LowerHex).
• Any HMAC change has to be proven output-identical before it ships — EC IDs and S3 SigV4 signatures are persisted, and a silent digest change would invalidate every existing identity.
• Three releases of new clippy lints arrive at once under the workspace's -D warnings gate.
• Docs and scripts that name specific tool versions need updating.

Done when

• All six CI gates pass on 1.98.1
• HMAC output verified unchanged against 0.12
• No new #[allow] suppressions introduced to get there
• Docs and scripts name the new versions