feat: background placement for anytime tasks

[Scofield626]

An anytime: node can now set background: true. It compiles to CuAsyncTask<CuAnytimeRunner<T, Policy>, O>: the node keeps a single Whole poll step in the plan, and the worker thread runs one whole job per dispatch — age check, base(), refine quanta under the policy ZST, finish.

• CuAnytimeRunner (doc(hidden), like the rest of the anytime machinery) holds the refinement loop in library code; every policy check const-folds exactly as in the emitted foreground blocks, with the same one-clock-read-per-quantum shape and stop-cause attribution.
• AnytimePolicy::MAX_REFINES is read only by the runner's loop; a foreground plan still encodes the count positionally and never reads it.
• The shared anchor_from_tov helper replaces the emitted anchor match; the foreground base block now calls it too.
• Codegen composes the wrapper type through one background_inner_type helper at every background type site; background anytime nodes keep their single plan step (no expansion, no hoisted job local).
• A background node does not need max_refines: a time bound alone closes the loop, which config validation already guarantees.
• Inherited from CuAsyncTask: job-final delivery at the first poll at or after the worker finished; the input clone preserves tov, so max_age_ms absorbs upstream delay and pool queueing; task errors surface at the next poll; freeze cannot happen mid-job.

Tests: runner unit tests for every stop cause (converge, quanta bound, budget, stale skip, abort at base, quality floor), a composition test through CuAsyncTask asserting the status stamp survives the buffered-output copy, compile-pass apps (default and sim mode) covering a time-only background node and a named pool with resources, and a runnable example with foreground and background nodes sharing one task type.

[github-actions]

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