docs(cas): update leftValue.name validation guidance

[gonzalezw]

Summary

A recent tdx release added client-side validation for a composable segment rule's leftValue.name — tdx cas sg push and tdx cas validate now catch a mismatch (e.g. using the raw CDW column name instead of the audience's declared attribute name) before it reaches the backend, instead of silently pushing a segment that matches zero rows.

This skill still described the old no-validation behavior as current/permanent in several places, which would give an AI agent (or human) an actively wrong mental model — e.g. telling someone "there's no way to catch this" when the CLI now does.

Changes

• known-limitations frontmatter: updated to describe the new structural + leftValue.name validation, and the scope limit (unrecognized condition types still pass through by design).
• The leftValue.name explanation: now describes when the check runs (push always resolves the live audience; offline validate only when a sibling audience.yml is present) and the remaining gap (a lone segment file with no sibling audience YAML).
• The "no schema validation ahead of push" paragraph: corrected to describe what's now checked vs. what's deliberately left unvalidated.
• Troubleshooting table: split the old "blank rule / 0 rows" row into the now-caught case (clear error) and the remaining uncaught case (lone file, no sibling audience), plus updated the "unfamiliar low-level error" row.
• Bumped last-validated.

Test plan

• Read through the full file for any other stale references to the old behavior — none found.