Fix #46: size the rejection-sampling mask without float overflow

[corgab]

Summary

EntropyGenerator::integer() sized its rejection-sampling chunks with (int) ceil(log($range + 1, 2)) and masked them with (1 << $bitsNeeded) - 1. Two things break near the top of the 64-bit range:

• (1 << 63) - 1 overflows to a negative float, so integer(0, PHP_INT_MAX) used a corrupted mask, as the issue reports.
• The float logarithm loses precision above 2^53: for range = 2^62, 2^62 + 1 rounds to 2^62 as a double, log2 gives exactly 62, and only 62 bits were drawn, so the top value of the range could never be produced. Verified: (int) ceil(log(2**62 + 1, 2)) is 62 while the range needs 63 bits. The issue's exhaustive check missed this because it covered ranges of the form 2^k - 1.

Plan

Derive the bit count with integer arithmetic, drop the mask (every chunk is exactly $bitsNeeded binary digits, so bindec() is already in range), and reject the one input the algorithm cannot serve: a span that overflows the subtraction, which would need a 64-bit chunk that bindec() returns as a float.

Changes

• EntropyGenerator::integer(): the overflow is detected before subtracting ($min < 0 && $max > PHP_INT_MAX + $min), which keeps $range an int for PHPStan and avoids relying on float promotion; $bitsNeeded = strlen(decbin($range)); the mask is gone. Docblock states the accepted bounds.
• README entropy section: one sentence on the accepted span.

The new guard throws \InvalidArgumentException, like the two argument checks already in this class (min > max, bits < 1): these are caller mistakes on plain integer arguments, not package failures, so they stay outside the AetherException hierarchy on purpose.

No config change, no new dependencies, no change for ordinary ranges (the bit length is identical below 2^53 and the mask never altered a value).

Tests

EntropyGeneratorTest, under a "Wide ranges" banner: integer(0, PHP_INT_MAX) returns PHP_INT_MAX from an all-ones chunk; integer(0, 2 ** 62) returns 2 ** 62 from a chunk of 1 followed by zeros (fails on main, which returns 2 ** 61); integer(PHP_INT_MIN, PHP_INT_MAX) throws before touching the device. The guard was also checked by hand on (PHP_INT_MIN, 0), (PHP_INT_MIN, -1), (0, PHP_INT_MAX) and (-1, PHP_INT_MAX).

Local: pint passed, pest 794 tests passed.

Closes #46