Add a size budget to concrete path execution

[isPANN]

Summary

Running candidate paths on an instance has no size or time limit. Some reductions produce huge targets from small inputs, and pred path <src> <dst> <instance> then hangs or runs out of memory. This is the local, reproducible form of #1152 (timeouts when MIPLIB instances are reduced toward a solver).

In a sweep of all variants' canonical examples (--limit 20), 8 runs timed out after 30 s and 1 was killed (probably out of memory). Timing individual prefixes (pred reduce --via):

Step	Input bundle	Output
MinimumCoveringByCliques → ILP	41 KB	600 MB
TravelingSalesman → ILP	21 KB	13 MB, then ILP → QUBO > 20 s
LongestCommonSubsequence → MIS	~1 KB example	61 MB
MinimumSetCovering → ILP (from the LCS→MIS output)	37 MB	> 20 s
MVC → LCS → MIS (from a SAT example)	35 KB	> 20 s

execute_paths also keeps every shared prefix's intermediate problem in memory until the whole batch finishes.

Proposal

• Add a size budget to instance execution (for example a maximum on the target's parameters or on total variables and constraints), checked after every step. Stop a path that exceeds it and report it as stopped over budget, alongside executed and failed.
• Where a step's symbolic contract is exact, or an upper_bound that is already within budget, use it to decide before building the target. That needs parameters to be available (Make reduction size formulas available end-to-end (ILP → QUBO first) #1175).
• Free intermediate results once no remaining path needs that prefix.
• Expose the budget in the CLI and MCP with a sane default.
• Tests with a deliberately blowing-up path, kept under the 5 s test limit.

Depends on: #1177 (per-path failure isolation) and #1175 (available size formulas, for the pre-build check). Feeds into #1179 (path ranking).