Add open variable lists - #248

Construct boards through an OpenIntVarList path transition and compare
with a bounded array model using the same slide and goal rules. Minimize
moves with BAB, and expose independent scramble, seed, and move-limit
options. Keep the default move limit at twelve for a short demonstration.

Register the example in CMake and Autoconf, including its generated
include dependencies, and document state-versus-move length accounting.

Validated both models in Debug and ASan/UBSan builds with the defaults,
an explicit scramble/limit/seed case, a solved start, and an insufficient
move bound. Replayed every printed path to verify legal slides and the
goal board. Independent Astra review passed.

Wait for the current board to become assigned before closing or extending
the path. Keep callback captures limited to the state index and retrieve
the model from the supplied space. Remove the initial Manhattan lower
bound on length to avoid constructing future boards at the root.

Both models pass small explicit, solved-start, and insufficient-limit
cases in Debug and ASan/UBSan builds. Printed paths were replayed, and a
root diagnostic confirms two constructed boards for scramble20/limit32,
seed3. Independent focused review passes.

The open default now exceeds a 30-second Debug cutoff because delayed
goal detection removes early backward goal propagation. Its usability
needs a separate coordinated follow-up; no extra pruning rule is included
in this model correction.

Reject assigned boards whose Manhattan distance cannot fit the current
path upper bound. Recheck this condition after incumbent cuts, taking
care not to read a board before propagation assigns it. Force the goal
when a constructed board reaches the final permitted state.

Keep path growth lazy: these checks do not increase its minimum length
or construct future boards. The fixed model and shared slide rules stay
unchanged.

Focused Debug and ASan/UBSan runs pass for defaults, explicit options,
a solved start and an insufficient move limit. Printed paths were
replayed. The default open model completes in 39 nodes, and the root
probe still constructs two boards for scramble20/limit32/seed3.
Independent Astra recheck passes.