Extend -size units to E + reject over-64-bit / unknown units

[helly25]

Summary

Review feedback on the freshly-added kSizeUnits map: complete the -size unit scale, and stop silently no-matching invalid units.

• kSizeUnits gains T (2^40), P (2^50), E (2^60). T/P are find-native (BSD find accepts up to P; verified). E is an xff continuation of the same binary scale -- a strict superset that changes the meaning of no find-valid input -- so all three are valid in every flavor, including --config=find (ungated; they're continuations, not conflicts). Z/Y/R/Q (2^70+) can't be added: they overflow the 64-bit byte count.
• -size arguments are now validated. MatchesSize is refactored onto ParseSizeSpec, and a new ValidateSizeArgs walks the expression; the driver runs it before traversal. An over-64-bit unit (Z/Y/R/Q) is rejected naming the limit -- "the largest size unit is E (exabyte)" -- and an unknown unit is rejected too, both exit 2. Previously a bad -size silently matched nothing; real find errors at parse time, so this is also more find-faithful.

$ xff . -size +1Z
xff: -size: '+1Z': size unit 'Z' exceeds xff's 64-bit byte range; the largest size unit is E (exabyte)
$ echo $?
2

Tests

• evaluate_test: SizeMatchesLargeUnits (T/P/E with exact multiples), ValidateSizeArgsRejectsBadUnits (valid units pass; Z and unknown rejected with messages).
• run_test: OversizedSizeUnitIsRefusedBeforeTraversal (exit 2, names the limit, no traversal).
• Verified on the binary: T/P/E accepted incl. under --config=find; +1Z and 1q error with exit 2.

Green on default and --config=asan; clang-format + pre-commit clean.