Replace Scenario scaling with iterative factory balancing

[ApocDev]

Problem

Factory Scenario currently scales cached block boundary rows. A valid producer whose current goal is zero therefore contributes an all-zero row and makes a positive target infeasible. More broadly, users want a direct Balance factory operation rather than an LP tied to the current nonzero projection.

Direction

Model the factory after YAFC-style iterative balancing:

• keep enabled terminal positive/stock goals pinned as the factory anchors;
• adjust intermediate positive goals to cover downstream demand;
• adjust explicit negative goals to absorb available surplus;
• re-solve each changed block and recompute factory boundary flows;
• repeat until stable, with tolerance/pass/no-progress guards;
• treat unsupported ingredients as external imports;
• probe valid zero-rate producers at a normalized nonzero rate so they can be activated;
• surface missing producers and non-convergence clearly.

Acceptance criteria

• Block 34-style zero-rate producers can be activated by factory balancing.
• Current terminal goals remain unchanged unless the user explicitly overrides them.
• Intermediate production and negative consumption goals settle across multiple blocks.
• Apply is one undoable action and does not partially persist failed iterations.
• Scenario remains a preview, with Balance factory as the primary current-target action.
• Solver, orchestration, browser, and documentation coverage pass.