populace-nz: add NZ wealth margins so encoded asset tests become operative (AS/TAS)

The rules side is already done: rulespec-nz's Accommodation Supplement module encodes the cash-asset limits (Social Security Regulations 2018 regs 15–19) and gates eligibility on accommodation_supplement_cash_assets_within_limit. What makes that gate operative in a take-up estimate is the population side: donor microdata carries wealth variables, but docs/nz-calibration-targets.md currently has no wealth rows, so cash assets would pass through donor-shaped rather than NZ-shaped.

Add NZ net-worth margins to the target inventory and calibration:

• Stats NZ household net worth statistics (HES net-worth module, triennial): net worth by quintile, age group, and household type; asset-class splits (property vs financial). Same portal caveats as the rest of the inventory — prefer the file CDN paths.
• Calibrate the donor wealth margins to these, alongside the existing income/demographic/tenure fits, so the joint income–wealth structure is disciplined by published NZ data.

Why it matters for the first published estimate: ignoring asset tests overstates AS eligibility and therefore the take-up gap — the exact overclaim a skeptical reviewer would attack first. It also flips the IDI limitation into an outside-in advantage: the IDI largely lacks asset variables, so an inside-the-lab analysis cannot apply the statutory asset tests — the calibrated open model can, and its wealth margins validate against the published Stats NZ distributions.

Part of the populace-nz epic (#343); pairs with the take-up engine plan's validation gates.

Read docs/nz-calibration-targets.md and the existing income, demographic, and tenure target rows first. Use the Stats NZ HES net-worth module, preferring file CDN paths, to add the listed net-worth and asset-class margins, then locate the calibration entry point that consumes the inventory. Done means wealth margins are calibrated alongside existing fits and validated against published NZ distributions.