Avoid overflow when aligning large frame counts

[morluto]

Problem

Frame requests are aligned upward to the next legal 5 + 17n shape. The calculation currently uses signed int arithmetic even though CLI and library inputs can reach INT32_MAX.

For INT32_MAX, the required calculation is:

remainder = (2147483647 - 5) % 17 = 3
increment = 17 - 3 = 14
aligned   = 2147483647 + 14 = 2147483661

The result cannot fit in an int. In an executed probe compiled from the upstream helper, it wrapped to -2147483635 instead of producing a valid aligned count.

Change

Perform the alignment arithmetic in int64_t. If the next legal frame count cannot be represented by the function's int return type, return zero as an explicit failure sentinel. Generation parameter validation now rejects that sentinel with the existing frame-range error.

Zero is unambiguous because every valid aligned frame count is at least 5.

Regression coverage

The host test now checks that aligning INT32_MAX returns zero. The same probe compiled from the candidate helper returns zero, while the upstream helper fails that expectation.

Validation

• actual-helper A/B probe: upstream -2147483635, candidate 0
• strict C syntax checks for h3.c and tests/test_h3.c — passed
• git diff --check — passed