fix(json): classify metadata carriers by kind

[denisvmedia]

Description

Attaching errx.Attrs() or errx.FromAttrMap(nil) currently produces "sentinels":["(empty attribute list)"]. JSON now identifies attribute and trace carriers by their own type, so empty metadata no longer becomes a sentinel. Real sentinels remain visible beside empty metadata and when their parents carry attributes or traces.

Fixes #57

Type of Change

• Bug fix (non-breaking API change)

Changes Made

• Exclude classifications implementing Attrs() []errx.Attr or stacktrace.Tracer, including external implementations with empty payloads.
• Preserve a sentinel's own text when its parent carries attributes or traces.
• Keep displayable detection chain-wide; document this distinction in the helper and JSON README.
• Replace the narrow empty-attributes test with a 56-case matrix covering classified, wrapped, standalone, and mixed errors. Assert exact sentinel lists, field omission, preserved messages, attributes, traces, and display text.

Testing

The regression matrix failed against master for empty metadata and sentinels with metadata parents, then passed with the fix.

• go test -race -count=1 -coverprofile=/private/tmp/errx-57-coverage.out ./... — all packages pass; 95.4% overall, 93.7% JSON, 100% for isPureSentinel.
• go test -race -count=1 ./json after the final fixture-only style adjustment.
• go vet ./....
• golangci-lint run --allow-serial-runners (local v2.14.0; CI pins v2.13.2).
• govulncheck ./... — no vulnerabilities found.
• go mod tidy -diff, gofmt, and git diff --check.

Documentation

• Updated the helper comment and json/README.md.
• Added an Unreleased CHANGELOG entry and the json: empty attribute sets are serialized as a sentinel #57 link definition.

Breaking Changes

No public signatures or JSON schema change. The intentional output corrections are:

• Empty attribute/trace carriers contribute no sentinel text, including foreign classifications whose only content is their Error() text. A standalone carrier still retains that text in message.
• A sentinel with an attributed or traced parent retains its own sentinel text. A displayable parent continues to exclude it, matching existing displayable behavior.

This does not add extraction support for external attribute providers; that remains separate from recognizing their kind.

Performance Impact

Two metadata traversal calls in isPureSentinel are replaced with direct interface checks. No quantified performance claim is made.

Checklist

• Read CONTRIBUTING.md and followed project conventions.
• Added regressions and completed local validation.
• Updated documentation and changelog.

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📥 Commits

Reviewing files that changed from the base of the PR and between 962602e and 811082b.

📒 Files selected for processing (5)

• CHANGELOG.md
• json/README.md
• json/json.go
• json/json_test.go
• json/sentinel_kind_test.go

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