CP-SAT knobs: worker env, domain tightening, solver params, trace capture, secondary costs

From the d=8192 scheduling campaign (2026-06-09/10):

- TW_CPSAT_WORKERS env (default 16, unchanged): the 64-CPU Modal compile
  container sets 64; confirmed num_search_workers reaches ortools.
- tighten_domains: per-node [earliest, latest] layer domains via
  mode-aware propagation (per-sublayer earliest/latest; matches CP-SAT
  presolve's own bound exactly — proves the d8192 critical path = 50).
  Saves ~7s presolve + ~9s to first incumbent on the DOOM graph.
- critical_path_layers(): public exact dependency-depth bound.
- solver_params: generic CpSolver parameter overrides for experiments.
- solution_trace: per-incumbent full-assignment capture (the basis of
  the trajectory analysis that killed secondary objectives).
- Costs.earliness / Costs.waste: lexicographic secondaries, kept as
  MEASURED FAILURES — trajectory data shows waste RISES as layers fall
  (compaction stretches lifetimes) and earliness rewards irrelevant
  early-region shuffling; docstrings record the mechanism so they are
  not re-proposed.  objective_scale on BuiltModel/SolveStats recovers
  the primary value.
- NOTE: ortools 9.15 IntVar.Proto()/.proto returns corrupted memory
  (segfault); domain bounds are tracked at var creation instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: physicsrob

docs/cpsat_scheduler.md

@@ -604,8 +604,9 @@ correct for every tested geometry).
 
 ### Determinism
 
-CP-SAT runs with `num_search_workers=16` and uses parallel worker
-strategies. Different runs may produce different `ScheduleAssignment`
+CP-SAT runs with `num_search_workers=16` by default (override with the
+`TW_CPSAT_WORKERS` env var — e.g. the 64-CPU Modal compile container
+sets it to 64) and uses parallel worker strategies. Different runs may produce different `ScheduleAssignment`
 values for the same model — different worker discovery orders find
 different optima of equal objective value. The compiled
 `HeadlessTransformer` differs across runs only in scheduling: token