[Certora] verify stayHealthy on take

[prd-carapulse]

Summary

• split Healthiness take coverage into seller-on-target-obligation vs other-borrower cases
• add a take seller helper and explicit take rules instead of leaving take excluded from stayHealthy
• relax callback-local health assertions only for the tracked seller case, while keeping end-of-take health checked

Test Plan

• forge build
• forge test --match-path test/TakeTest.sol --match-test 'test(BuySellerCallback|SellSellerCallback|SellSellerCallbackLiquidateRevertsWhileLiquidationLocked|BuyUnhealthy|SellUnhealthy)' -vv
• PATH="/tmp/jre21/bin:/home/hermes/.svm/0.8.31:$PATH" certoraRun certora/confs/Healthiness.conf --compilation_steps_only

Stacked on #645.

[MathisGD]

see #679