test(errors): cover Wrap/Multi/Join/Is* and category inference (100%)

[abhicris]

Summary

The github.com/luxfi/node/errors package shipped with zero tests despite being a public utility surface (sentinel errors, WrappedError context chain, Multi/Join aggregation, retry helpers like IsTemporary/IsPermanent). This adds 22 tests that bring the package to 100.0% statement coverage.

What is covered

• WrappedError.Error() format — with and without an explicit message
• Wrap / WrapWithContext nil pass-through; context map preservation; allocated-and-writable context on the bare Wrap path
• errors.Is / errors.As chain through Wrap (including double-wrap)
• IsNotFound, IsClosed, IsTimeout direct + %w wrapping
• IsTemporary against both the sentinel set and the optional Temporary() bool interface
• IsPermanent across every permanent sentinel + fmt.Errorf %w chains
• GetCategory: wrapped category, sentinel inference, unknown fallback, and the precedence rule (wrapped category beats inferred)
• Multi.Error / Add / Err at sizes 0, 1, and N; nil-add ignored
• Join nil pass-through and multi-error combination

Verification

$ go test ./errors/... -count=1 -v
... 22 PASS ...
PASS
ok  	github.com/luxfi/node/errors	0.008s

$ go test ./errors/... -race -count=1
ok  	github.com/luxfi/node/errors	1.020s

$ go test ./errors/... -cover
ok  	github.com/luxfi/node/errors	0.011s	coverage: 100.0% of statements

$ go vet ./errors/...
(clean)

$ go build ./errors/...
(clean)

Notes

• Pure test-only change — no edits to errors/errors.go.
• License header matches header.yml.
• No collision with the docs PR docs: align Go version in README and CONTRIBUTING with go.mod (1.26.3) #125.