docs: find-flavor compatibility design (POSIX/GNU/BSD union)

[helly25]

What

Records the design-of-record for xff's find-compatibility target, answering "which find do we target, and do we need more than one flavor?"

Decision: one permissive find style = POSIX (original) ∪ GNU ∪ BSD, not separate find-gnu/find-bsd flavors.

Why

• xff already implements the union - it ships BSD's birthtime (-Bmin/-Btime) and GNU's -printf/-mmin/-regextype. Two strict flavors would mean retracting features from each.
• The divergence is almost entirely additive (different spellings for similar capabilities), so the union is a strict superset and every GNU/BSD/POSIX script keeps working.
• The find vs xff line is about xff inventions (-println, -capture, {field}, compound durations), not GNU-vs-BSD spelling.

Contents

• Divergence survey table (probed against real /usr/bin/find BSD + GNU findutils; the dev-box find is bfs, a superset).
• Resolutions for the few same-syntax-different-meaning conflicts (-type f,d, -perm +/-//, -size suffixes, regex flavor).
• Settles the time-predicate gating: -mtime unit suffixes (-1h, +2d) are BSD-native -> find-compatible (not xff-gated); only compound durations on -mtime are xff-only.

Design-of-record for the rich time-comparison work (compound ParseTimeString + -mtime unit suffixes) that follows in stacked PRs.