feat: add math functions SIGN/CBRT/TRUNC

[YinZiBenRun]

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.

Resolves #790

Changes

New UDF classes

• Sign.java — sign(x) returns -1 for negative, 0 for zero, 1 for positive. Supports Double/Long/Integer input.
• Cbrt.java — cbrt(x) returns the cube root of x. Supports Double input.
• Trunc.java — trunc(x, d) truncates x to d decimal places (not round). Supports Double/Long/Integer input with
  optional scale parameter.

Registration

• Added Sign, Cbrt, Trunc to BuildInSqlFunctionTable.java
• Added cbrt(Double) and trunc(Double, Integer) static methods to GeaFlowBuiltinFunctions.java (sign already
  existed)

Tests

• Added MathUdfTest.java — unit tests for Sign, Cbrt, Trunc UDF classes
• Added testCbrt() and testTrunc() to InternalFunctionsTest.java
• All tests pass: 20/20

TRUNC vs ROUND — key distinction

trunc uses RoundingMode.DOWN while round uses RoundingMode.HALF_UP:

-- trunc: truncate (no rounding)
SELECT trunc(3.1465, 2)  →  3.14
SELECT trunc(-3.1465, 2) →  -3.14

-- round: round half up
SELECT round(3.1465, 2)  →  3.15
SELECT round(-3.1465, 2) →  -3.15

Edge cases covered

┌──────────┬─────────────────────────────┬────────┬─────────┐
│ Function │          Negative           │  Zero  │  Null   │
├──────────┼─────────────────────────────┼────────┼─────────┤
│ sign     │ ✅ -1                       │ ✅ 0   │ ✅ null │
├──────────┼─────────────────────────────┼────────┼─────────┤
│ cbrt     │ ✅ -2.0 (cbrt(-8))          │ ✅ 0.0 │ ✅ null │
├──────────┼─────────────────────────────┼────────┼─────────┤
│ trunc    │ ✅ -3.14 (trunc(-3.1465,2)) │ ✅ 0.0 │ ✅ null │
└──────────┴─────────────────────────────┴────────┴─────────┘

[YinZiBenRun]

#790

[YinZiBenRun]

#790