Bug: std.format %g precision threshold and zero handling

## Description

The `%g` format specifier in `std.format` has two bugs related to precision handling. According to the Jsonnet documentation, `std.format` should follow Python's `%` formatting rules.

## Bug 1: Incorrect threshold for exponential notation

**Expected behavior**: When the exponent equals the precision, `%g` should use exponential notation.

**Test case**:
```jsonnet
std.format("%.6g", [1e6])
```

**Python reference**:
```python
>>> "%.6g" % 1e6
'1e+06'
```

**go-jsonnet output**: `"1000000"` (incorrect - uses fixed-point notation)

**Expected output**: `"1e+06"` (exponential notation)

The threshold should be `exponent >= precision`, not `exponent > precision`.

## Bug 2: Incorrect zero handling with precision 0

**Expected behavior**: When formatting `0.0` with precision 0 using `%g`, the output should be `"0"`, not `"0e+00"`.

**Test case**:
```jsonnet
std.format("%.0g", [0.0])
```

**Python reference**:
```python
>>> "%.0g" % 0.0
'0'
```

**go-jsonnet output**: `"0e+00"` (incorrect)

**Expected output**: `"0"`

## Additional test cases

```jsonnet
// Bug 1: threshold tests
std.format("%.1g", [10]) // Expected: "1e+01", go-jsonnet: "10"
std.format("%.2g", [100]) // Expected: "1e+02", go-jsonnet: "100"
std.format("%.3g", [1000]) // Expected: "1e+03", go-jsonnet: "1000"

// Bug 2: zero with different precisions
std.format("%.0g", [0.0]) // Expected: "0", go-jsonnet: "0e+00"
std.format("%.1g", [0.0]) // Expected: "0", go-jsonnet: "0e+00"
```

## Verification

All test cases have been verified against Python 3's `%` operator, which is the reference implementation according to the Jsonnet specification.

## Impact

These bugs affect compatibility with Python's formatting behavior, which is the documented standard for `std.format`.

Locate the std.format implementation and its existing tests, then run the reported Jsonnet cases for %.6g, %.0g, and the additional threshold and zero examples. Compare the results with Python's % formatting behavior; done means the cases use exponential notation at the stated threshold and format zero as "0" without an exponent.

Written by the indexing model from the issue text.