Restore `scorp → s_corp_income` mapping (revert #1051's change back to the pe-us #8613 leaf)

#1051 changed scorp → partnership_s_corp_income, which reverted #1042 (scorp → s_corp_income, mapped to the dedicated S-corp leaf split out in pe-us #8613). #1051's rationale — "holding the leaf suppressed QBID" — only applied with the qualified_business_income holdout present, which #1051 itself removed.

Verified: once #1051's holdout fix is in place, scorp → s_corp_income yields the correct QBID (16,850, including the scorp + pbusinc case), with identical federal/CA/eCPS results. So the revert is unnecessary and mapping to the parent (partnership_s_corp_income) risks leaving the s_corp_income leaf unpopulated for any state calc that references it specifically.

Fix: map scorp → s_corp_income (one line in variable_mappings.yaml + the two mapper paths), restoring the #1042/#8613 coordination. Low priority — empirically harmless — but keeps the leaf correct. From the #1051 review.

Start in variable_mappings.yaml and inspect the two mapper paths mentioned in the issue, along with the surrounding scorp and s_corp_income mappings. Restore the mapping to the dedicated leaf, then verify the scorp and scorp + pbusinc cases produce the stated QBID and retain identical federal, CA, and eCPS results.