deps: bump dtolnay/rust-toolchain to current stable

[Verdenroz]

What changed

Every dtolnay/rust-toolchain pin moves from the August 5 stable commit to the current one, 6bed0761d98439e5a578e2877258200ad565ba87 (September 3). Seventeen uses: lines across the workflows, nothing else.

Why

dtolnay rewrites the stable branch rather than appending to it, so the commit the workflows pinned is no longer reachable from any branch or tag in that repository. zizmor's online impostor-commit audit treats such a pin as an error, and the code-scanning check it feeds went red on #165 for that reason alone. The same alert will fire on master and on every stacked PR at its next Security run, so this lands first and the stack rebases onto it.

How was this tested

• Verified 6bed0761d98439e5a578e2877258200ad565ba87 is the head of dtolnay/rust-toolchain@stable via the GitHub API.
• No other change; CI on this PR is the test.

Checklist

• make check passes (fmt, clippy -D warnings, cargo test --workspace)
• make gate BASE=master passes, or any intentional cost change is explained above
• Tests added/updated for new behavior (not required for docs/CI-only changes)
• Public API items have /// doc comments
• No new dependency, or its justification is included above (see Dependency Policy)
• Docs (README.md, docs/, soothfast:bind/soothfast:claim markers) updated if behavior changed

[soothfast-bot]

soothfast gate

gate: measuring merge-base of origin/master in worktree (interleaved rounds)
gate: bench binaries identical (code and data match) — no measurable change possible
gate: gating backend = walltime
gate: build=08f03a noise_floor=0.23% thresholds: instructions +5% ir +5% walltime +10.0% alloc/size +5% polls/wakes +5%
ok    soothfast_spec::bench_openapi_diff walltime_median_ns 5578266.0 -> 5578266.0 (+0.0%)
ok    soothfast_spec::bench_openapi_diff allocs 77722 -> 77722 (allowed <= 81608)
ok    soothfast_spec::bench_openapi_diff alloc_bytes 10815487 -> 10815487 (allowed <= 11356261)
ok    soothfast_spec::bench_openapi_document walltime_median_ns 1930648.0 -> 1930648.0 (+0.0%)
ok    soothfast_spec::bench_openapi_document allocs 30152 -> 30152 (allowed <= 31659)
ok    soothfast_spec::bench_openapi_document alloc_bytes 4715065 -> 4715065 (allowed <= 4950818)
ok    soothfast_spec::bench_serialize_yaml walltime_median_ns 6456474.0 -> 6456474.0 (+0.0%)
ok    soothfast_spec::bench_serialize_yaml allocs 74277 -> 74277 (allowed <= 77990)
ok    soothfast_spec::bench_serialize_yaml alloc_bytes 8533615 -> 8533615 (allowed <= 8960295)
ok    soothfast_spec::bench_openapi_diff assert complexity: claimed O(n); growth drift x1.43 over sizes [16, 64, 256] (limit x2.5)
ok    soothfast_spec::bench_openapi_document assert complexity: claimed O(n); growth drift x1.51 over sizes [16, 64, 256] (limit x2.5)
ok    soothfast_spec::bench_serialize_yaml assert complexity: claimed O(n); growth drift x1.18 over sizes [16, 64, 256] (limit x2.5)
gate: passed (3 item(s))