feat: add event stringers

[besmpl]

Summary

• add MouseButton.String names consistent with the newer pointer Button type
• add deterministic Modifiers.String output in the same order used by gogpu/ui
• share existing directional string constants so the package remains lint-clean
• cover every button, individual and combined modifiers, locks, zero, and unknown bits

Why

The public event enums should be directly useful in logs and diagnostics. Key already implements fmt.Stringer; this completes the remaining event Stringer slice from the API audit without changing type layout or event behavior.

Verification

• go test ./...
• go test -race ./...
• go build ./...
• go vet ./...
• golangci-lint run --timeout=5m
• go test -covermode=atomic -coverprofile=coverage.out ./... — 90.0% total statement coverage and 100% of changed production statements (28/28; no uncovered changed blocks)
• gofmt and git diff --check

CI note: GitHub marks this fork pull request's workflow run as action_required; upstream Actions and the Codecov upload will start after a repository maintainer approves the run.

Addresses the MouseButton/Modifiers Stringer item in #19; the broader API audit remains open.