feat: add _where filtering, use new op separator, drop _start, _end, _limit (#1696)

* feat: add _where filtering and new op separator

* Restore original README content above Query capabilities overview (#1697)

* test: refactor service tests to lightweight table cases

* chore: document underscore where operator compatibility

* refactor: clarify blank-string handling in where coercion

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Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>