WGPU/Metal backend, pure-env config loader, nextest migration (MULTI-1407)

[RobbieMcKinstry]

Summary

Three stacked commits on trunk:

• 043116d — Add WGPU/Metal backend with backend-aware precision (MULTI-1407). New wgpu crate feature; selection order is cuda > wgpu > ndarray. Precision is now backend-aware: bf16 on CUDA (the locked cloud recipe — MULTI-1386), f32 on WGPU (Metal has no bf16 arithmetic over WGPU) and ndarray. Crate-root #![recursion_limit = "256"] bump for CubeCL's #[cube] macros. Forward/backward smoke test on the active backend, verified locally on Metal with cargo test --no-default-features --features wgpu.
• a6240ce — Make the config loader pure of std::env; assert no-flake testing policy. Two pre-existing loader tests flaked under default-parallel cargo test because figment::Jail::set_env mutates process-global env via std::env::set_var while concurrent tests (without a Jail) read WUBBIE_MODEL_* through figment::Env::prefixed. LayeredConfig::with_env no longer reads process env; read_model_env_overrides() captures std::env::vars() once at the CLI boundary into an EnvOverrides map that is threaded through load_model_config / TrainSubcommand::resolve_model_config. Tests build the map directly and never mutate process env. CLAUDE.md gains a Testing policy section codifying the no-flake rule and the read-ambient-state-at-the-CLI-seam pattern.
• 57be649 — Migrate test runner to nextest; add bacon-driven monitor task. CI swaps cargo test for cargo nextest run --workspace --locked --no-tests=pass; Makefile.toml gets matching test and monitor tasks (the latter driven by bacon --headless).

Acceptance criteria — MULTI-1407

• Backend selectable (CUDA / WGPU-Metal / ndarray) without model-code changes — ✅
• Builds cleanly with the wgpu feature (#![recursion_limit = "256"] applied) — ✅
• Precision is backend-appropriate: f32 under WGPU/Metal, bf16 path preserved for CUDA — ✅
• Forward/backward step under WGPU/Metal — ✅ (smoke test runs on Metal locally)

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets --workspace --locked -- -D warnings (default ndarray, --features wgpu, --features cuda)
• cargo build --workspace --locked (default) and cargo build -p wubbie --no-default-features --features {cuda,wgpu} --locked
• cargo nextest run --workspace --locked --no-tests=pass — 55/55 deterministic across 5 consecutive runs
• WUBBIE_MODEL_D_FF=9999 cargo nextest run --workspace --locked --no-tests=pass — still 55/55 (confirms the loader is now pure of process env)
• cargo test --no-default-features --features wgpu --lib backend:: runs the forward/backward smoke on Metal via WGPU
• CI ⚡ PR Ready green (push, merge-queue contexts)

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