build: pin the exact Rust toolchain (1.96.0) to stop rustfmt drift

rust-toolchain.toml pinned the floating "stable" channel, so the CI image baked whatever
stable existed at image-build time. When the image is rebuilt onto a newer stable,
rustfmt's rules shift and `cargo fmt --all --check` fails on files nobody touched — the
recurring format-drift that keeps red-lighting CI.

Pin channel = "1.96.0" (== today's stable: rustc ac68faa20, rustfmt 1.9.0-stable), the
exact build CI already runs, so this is a no-op now but locks formatting for good: local
dev, the Linux CI image, and the Windows runner all use rustup and honor this file, so
they converge on one rustfmt. Formatting now only changes in a deliberate bump-this-pin-
and-reformat commit. ci.yml cache comment updated to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -28,8 +28,8 @@ jobs:
 
       # Best-effort caches (act_runner's built-in cache server). Keyed on Cargo.lock:
       # registry/git are download caches, target/ the incremental build. The target key
-      # carries the rustc version — rust-toolchain.toml pins the floating "stable"
-      # channel, so the file alone wouldn't invalidate stale incremental state.
+      # carries the rustc version — resolved via `rustc --version` (below) rather than parsed
+      # from rust-toolchain.toml, so a pin bump there invalidates stale incremental state too.
       - name: Cache keys
         run: echo "rustc=$(rustc --version | cut -d' ' -f2)" >> "$GITHUB_ENV"
       - uses: actions/cache@v4

rust-toolchain.toml

@@ -1,3 +1,9 @@
+# Pin an EXACT version, not the floating "stable" channel. The CI image bakes whatever
+# `stable` was at image-build time; when the image is rebuilt onto a newer stable, rustfmt's
+# formatting rules shift and `cargo fmt --all --check` starts failing on files nobody touched
+# (format drift). An exact pin makes rustfmt/rustc identical for every contributor and CI, so
+# formatting only ever changes in a deliberate bump-this-line-and-reformat commit.
+# The CI image (ci/rust-ci.Dockerfile) and the Windows runner both use rustup, which honors this.
 [toolchain]
-channel = "stable"
+channel = "1.96.0"
 components = ["rustfmt", "clippy"]