integration: add mockgrbl realtime hold/resume and status-report tests

[ianbruene]

Motivation

• Extend the Linux-only mockgrbl_e2e integration suite to cover realtime feed-hold/cycle-start behavior during active motion and to verify controller status/report consistency while motion is active and after completion.

Description

• Add TestDDGoRealtimeHoldResumeDuringJogAgainstMock which drives a long X jog via the real app.Controller, issues grbl.ActionHold and grbl.ActionResume, asserts the mock materializes a held/cancelled position between start and target, verifies the mock logged the realtime hold, and verifies resume does not restart the cancelled jog.
• Add TestDDGoStatusReportsDuringAndAfterMockJog which starts a short X jog, requests explicit status while the mock is moving and after natural completion, and asserts the controller snapshot shows Jog while moving, Idle after completion, and that LastStatusRaw is present.
• Add a small helper hasMockLogEntry to flexibly assert mock event/response text without brittle exact-match expectations, and keep the existing //go:build linux && serial && mockgrbl_e2e scope and the real serial transport.
• Only test-file changes were made to internal/integration/mockgrbl/mockgrbl_e2e_test.go; no production code or CI workflows were modified.

Testing

• Ran gofmt -w internal/integration/mockgrbl/mockgrbl_e2e_test.go and the file formatting check passed.
• Ran go test -tags 'serial mockgrbl_e2e' ./internal/integration/mockgrbl and the mockgrbl integration suite passed.
• Ran go test ./... and go test -tags serial ./internal/transport ./internal/ports ./internal/app ./internal/grbl and all targeted packages passed.

---

Codex Task