sccache-dist for OpenEmbedded/Yocto: upstreaming tracker

[jetm]

Tracks upstreaming the sccache-dist changes that make distributed compilation
work for OpenEmbedded/Yocto host-mode builds. The original single PR
(mozilla#2750, closed) grew to ~40 commits; this splits it into small,
independently reviewable PRs against mozilla/sccache, landed in waves.

All branches are rebased on current upstream main and build/test green
(cargo check --all-features --all-targets, full test suite, fmt, clippy).

Plan

Legend: fill the PR link as each opens. Lane A is independent; the scheduler work
is a base-first stack (9 -> 8a -> 8b -> 8c -> 8d -> 6 -> 7).

Wave 1 - credibility

• PR 1 server: don't panic on a signalless finished compile (dist-server-signalless-panic) - server: don't panic when a finished compile has neither code nor signal mozilla/sccache#2776
• PR 2 compiler: fall back to local on distributed-compile failure (compiler-local-fallback) - compiler: fall back to local on distributed-compile failure mozilla/sccache#2777

Wave 2 - fix surface

• PR 3 dist/pkg: toolchain packaging correctness (dist-pkg-toolchain-packaging, depends PR 5)
• PR 4 compiler/gcc: distributed-compile correctness and force-local policy (compiler-gcc-dist-correctness)
• PR 5 compiler/rust: make distributed compiles resolve std and --target (compiler-rust-dist-std-target)

Wave 3 - scheduler stack (base-first)

• PR 9 dist: report a per-server breakdown in scheduler status (dist-scheduler-per-server-status)
• PR 8a dist/scheduler: drop a stopped server promptly (dist-scheduler-drop-stopped-server, on 9)
• PR 8b dist/scheduler: topology-aware, deterministic routing (dist-scheduler-topology-routing, on 8a)
• PR 8c dist/scheduler: idempotent updates and lost-job reaping (dist-scheduler-idempotent-reaping, on 8b)
• PR 8d dist/scheduler: liveness-lease reaping and restart-safe accounting (dist-scheduler-liveness-lease, on 8c) - wire-format change
• PR 6 dist/http: reuse connections and offload large deserialize (dist-http-connection-reuse, on 8d) - new config keys + arc-swap
• PR 7 dist/server: shrink toolchain locks so compiles stop convoying (dist-server-toolchain-locks, on 6)

Wave 4 - optional / chore

• PR 10 dist: load and dispatch instrumentation (dist-load-instrumentation, depends PR 2, ask-first)
• compiler: concatenate strings without relying on deref coercion (compiler-string-concat-fix)
• ci: mirror the CI validation gate in pre-commit (ci-precommit-gate)
• chore: fix clippy lints flagged by a newer toolchain (chore-clippy-newer-toolchain)

Discussion anchor

Upstream RFC issue (scheduler arc + instrumentation ask): mozilla#2775.