docs/plans/budget-tree: execution plan (consumer-0 / Track A / Track B / actuator)

[gunbai-bot]

Adds §6 (execution plan) to the budget-tree grounding doc, so the path from the merged model to a protective wall is recorded before we execute Track A.

§§1-5 (already on main, bright-stag) describe what #5582 landed. §6 captures the next steps:

• consumer-0: instantiate the real srv1 fleet tree from measured peaks (CI measurement: rust_tests-job leaf cgroup peak + --measure-cgroup-peak (per-job placement divisor) #5574 leaf-cgroup), divide-once.
• Track A (immediate, in-tree, no fenced apply): wire std.realization_width spawn-width to the tree's L2 allocation -> an over-wide spawn that would OOM the floor becomes a compile error; dissolves one §3 fork.
• Track B (the stern-otter OOM cure; needs the actuator): derive CI run-concurrency R / compile-jobs N divide-once; PREVENTS OOM only paired with the operator-decided actuator (admission cap or cgroup memory.max, fenced).
• §3 convergence: forked budgets (realization_width, complexity EffortBudget) collapse onto extdeps.accounting.budget as consumers import allocations; roadmap re-homes §1-C: memory-aware CI floor spawn_width — derived from measured VmHWM peak + live budget (not pinned 4) #5444/CI cross-host placement model + runner-deploy-derivation (gunbc shapes, drift-gated; live srv1/srv2 apply fenced for operator) #5559/CI compile-jobs from the model (slice A): derive cargo build parallelism, fail-closed serial, live nextest-gate consumer #5546 as edge-children of 1-budget-tree.

Docs-only; edits an already-linked plan doc (no orphan/dangling).

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