v1.3.1

External review round 7 findings.

• Fixed NewSkip / WrapSkip with a skip large enough that 3+skip
  overflows int (e.g. math.MaxInt) recorded a bogus frame near the top
  of the call stack instead of the documented "unknown" fallback for a skip
  past the stack top. caller now guards the overflow explicitly.
• Changed Error's allocation drops back to 144 B (was 160 B since
  v1.3.0): noPublicBelow moved right after code in the struct, filling
  its padding byte instead of adding one at the end. Pure layout — no
  behavior or API change. Pinned by a struct-size test; README/doc.go
  benchmarks and cost notes updated.
• Changed LogValue no longer costs allocations for public fields and
  field violations it never emits — collect's internal signature gained
  an includePublic flag, false for LogValue, true for %+v. Same
  chain-walk count, just fewer discarded slices.
• Docs The "exactly three channels" wording missed by the v1.3.0 sweep
  — doc.go's package-overview bullet, DESIGN.md's header/struct
  sketch/test-plan lines, and an inspect_test.go comment — now says four.