Add the math functions `SIGN` / `CBRT` / `TRUNC`

[Leomrlin]

What & why

The math folder udf/table/math/ currently has only E, Log2, and Round —
noticeably thin. Adding SIGN (the sign of a number), CBRT (cube root), and
TRUNC (truncate to N decimals) is a quick win that meaningfully fills out the
SQL toolbox.

数学函数目录现在只有 E、Log2、Round 三个,明显偏薄。补上 SIGN(符号)、
CBRT(立方根)、TRUNC(截断到指定小数位),花不了多少时间,却能明显补全 SQL 表达力。

The task

• sign(x) → -1 / 0 / 1
• cbrt(x) → cube root
• trunc(x, d) → truncate (not round) to d decimal places

Where to look

1. Follow udf/table/math/Round.java; in the same folder add Sign.java,
   Cbrt.java, Trunc.java (extend UDF, add @Description).
2. Register them in BuildInSqlFunctionTable.java.
3. Add tests under query/ + expect/ in geaflow-dsl-runtime, covering
   negative numbers, zero, and null.

Done when

• Three functions implemented and registered
• Negative / zero / null edge cases covered
• trunc vs round clearly distinguished (truncate ≠ round) — show an example in the PR
• Tests pass

[adamdavis99]

can I take this and start working on it ?