feat: user-defined equality, patch apply, generics, and extensibility

[christiangda]

Summary

Implements the improvements identified in the API review, turning promised-but-missing behavior into real features and adding high-leverage capabilities. All additions are backward-compatible and remain stdlib-only (zero external dependencies).

What changed

🐞 Correctness / dead code

• Equatable[T] / Comparable[T] now work. A type's Equals(T) bool or CompareTo(T) int method is honored automatically (custom comparators still take precedence). They were previously exported and documented but never consulted.
• Unified diff now emits context lines. Replaced the placeholder with an LCS-based line diff producing proper context windows and consistent add/remove/context change types (the old code hard-coded _ = contextSize).
• WithEquateEmpty(bool) — a real toggle for nil/empty-container equivalence (the old EquateEmpty() was enable-only).

🚀 New capabilities

• ApplyJSONPatch — RFC 6902 applier (add/remove/replace/move/copy/test), closing the generate→apply loop.
• Type-safe generics — EqualT[T], DeepEqualT[T], DiffT[T], CompareWithDiffT[T], GetJSONPatchT[T].
• Context-aware comparison — EqualCtx/DiffCtx/CompareWithDiffCtx + ContextComparator, cancellable via context.Context.
• WithFieldNaming(JSONTagNaming) — JSON-tag-aware paths and patch pointers so patches apply to real JSON documents.

🔧 Extensibility & ergonomics

• WithReporter — stream each Difference as it's found.
• Richer ignores — WithIgnorePaths, WithIgnorePathPatterns (regexp), WithIgnoreMapKeys, and the comparator:"-" struct tag.
• RegisterFormatter — pluggable output formats (CSV, JUnit, SARIF, …). Unknown formats now return ErrUnknownFormat.
• String() helpers for UnifiedDiff and VisualDiff.
• Sentinel errors — ErrUnknownFormat, ErrCanceled, ErrInvalidPatch.
• Testing helpers — AssertEqual/AssertNotEqual/RequireEqual (via a minimal TestingT interface, so testing isn't pulled into production builds).

📖 Docs & AI guidance

• Updated README + docs/ and added Extensibility and Testing guides.
• Merged the slashdevops/e5t and ds-utils conventions into .github/copilot-instructions.md, tailored to this stdlib-only library, and exposed it as AGENTS.md (symlink). Ticket-number and no-AI-coauthor rules from ds-utils were intentionally omitted as they don't fit this open-source repo.

⚠️ Behavior change

FormatDiff with an unknown format now returns ErrUnknownFormat instead of silently falling back to text. The existing test was updated to assert the new contract.

Verification

gofmt -l .        # clean
go vet ./...      # clean
go build ./...    # ok
go test -race -covermode=atomic ./...   # PASS, 82.7% coverage
go list -deps ... # no external deps

🤖 Generated with Claude Code