Fix Print::printf return value on truncation and error

[l5yth]

• vsnprintf returns the would-be length (not bytes written) and a negative int on encoding error.
• Storing that in size_t and returning it gave callers an overcount on truncation and ~SIZE_MAX on error.
• Clamp to what actually fit and return write()'s byte count.
• Fix Print::printf returns an incorrect byte count #72

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Pull request overview

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This PR fixes Print::printf’s return value semantics by correctly handling vsnprintf’s “would-have-written” length on truncation and negative values on encoding errors (Fixes #72).

Changes:

• Store vsnprintf’s return value as int and treat <= 0 as no bytes written.
• Clamp the output length to the buffer’s actual written capacity on truncation.
• Return the byte count from write() instead of the vsnprintf would-be length.

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