Refine IL CHIP premium encoding for Level 1 child-count schedule

Follow-up from #8086.

## Context

The newly-added `il_chip_premium` encodes All Kids Premium Level 1 with a simple per-child rate + family cap pattern:

- per_child: $15
- family_cap: $40

This gives: 1 child → $15, 2 → $30, 3 → $40, 4+ → $40.

The actual [IL HFS schedule](https://hfs.illinois.gov/medicalprograms/allkids/income.html) for Level 1 is:

- 1 child: $15
- 2 children: $25
- 3 children: $30
- 4 children: $35
- 5+ children: $40

So a 2-child tax unit at Level 1 FPL is encoded $5/mo ($60/yr) too high. Level 2 ($40/child capped at $80) is correct.

## Why it didn't block the PR

IL's `chip_child_income_limit` is `-.inf` in `policyengine_us/parameters/gov/hhs/chip/child/income_limit.yaml`, so `is_chip_eligible_child` always returns False for IL residents in natural microsim — the formula silently returns 0. The issue only surfaces in synthetic test cases that set `is_chip_eligible_child: true` directly.

## Proposed fix

Either:

1. **Exact schedule**: replace `per_child.yaml` and `family_cap.yaml` with a single rate scale keyed on `n_chip_children` (similar to IN's `rate_one_child` / `rate_two_or_more_children` split, but with more brackets). Formula picks the rate for that child count.
2. **Piecewise formula**: keep per-child + cap, plus an "additional child rate" parameter for the $5 incremental structure: `premium = min(per_child + max(n - 1, 0) * extra_per_child, family_cap)` — but the IL source pattern doesn't cleanly fit this either (first-child $15, second-child +$10, subsequent +$5 each).

Option 1 is more faithful. Level 2 ($40/child capped at $80 = two-child max) can stay as-is since `per_child × cap` gives the right answer there.

## Related

While we're here, `is_chip_eligible` returning False for IL (and MI, which also has a separate-CHIP program) is an upstream eligibility gap — IL's All Kids Premium and MI's MIChild are separate-S-CHIP programs, not pure M-CHIP expansions. Filing separately may be worthwhile.

Start by reading per_child.yaml and family_cap.yaml for the IL CHIP premium parameters, then inspect the existing synthetic cases that set is_chip_eligible_child: true. Encode the Level 1 schedule for one through five or more children while preserving the current Level 2 behavior, and verify the resulting monthly premiums against the listed values.

Written by the indexing model from the issue text.