optimizer: emit an inspectable trace across extraction, candidates, and assignments #774

Goal

Provide an inspectable, machine-readable optimizer trace rather than relying on verbose logs alone.

Change

Add an option such as --optimizer-trace-json <path> that emits stage-tagged JSON for:

1. RQEs
2. extracted QueryRequirements
3. OptimizerItems (requirements, T, SLAs, frequency)
4. generated structural candidates per item
5. eligible (item, config) edges after active filters
6. selected deployments and item assignments

Keep normal debug! logs concise: stage counts and stable identifiers, with the trace artifact holding full details.

Integration

• Optimizer: print candidates for a workload and diff against sketch-bench #129 #767 consumes the item/candidate stages for diffing against sketch-bench Re-organize code in repo to be more OSS friendly #129.
• Optimizer: mip_assign (good_lp + HiGHS) and asap-optimizer-cli --solver greedy|mip #761 supplies the selected-config and assignment stages.
• The trace format should be stable enough for regression fixtures.

Part of #753.