saturate overflowing numeric string casts

[vapebw]

In php 8.x, casting a numeric string in float or scientific notation that exceeds the 64-bit integer range to (int) (or via intval()) saturates to PHP_INT_MAX (9223372036854775807) for positive overflow and PHP_INT_MIN (-9223372036854775808) for negative overflow

Previously, Value.parseLeadingInt() in src/runtime/value.zig returned 0 when float strings overflowed max_f, causing incorrect int conversions such as:

• (int)"1e100" returning 0 instead of 9223372036854775807
• (int)"-1e100" returning 0 instead of -9223372036854775808
• (int)"1e19" returning 0 instead of 9223372036854775807
• (int)"9.9e18" returning 0 instead of 9223372036854775807

While pure integer digit strings (such as (int)"99999999999999999999") already saturated correctly to maxInt(i64) / minInt(i64), the float/exponent path was missing this behavior and returning 0

Solution

• Clamp positive float overflow (f >= max_f) to std.math.maxInt(i64) in Value.parseLeadingInt
• Clamp negative float overflow (f <= -max_f) to std.math.minInt(i64) in Value.parseLeadingInt

Tests

• Added unit tests in src/runtime/value.zig covering positive overflow, negative overflow, boundary values, and in-range float conversions
• Added parity test in tests/numeric_string_int_saturation.php matching native PHP 8.x behavior