Add mockgrbl CI integration-test foundation

[ianbruene]

Motivation

• Provide a small, reliable CI foundation to exercise DDGo against the Linux PTY-backed mockgrbl so integration tests can run in CI without hardware or UI dependencies.
• Keep the change small and reviewable while proving the mock can be launched in CI, exposes a serial symlink and debug API, and supports minimal DDGo-over-mock tests.

Description

• Add a smoke script scripts/ci_mockgrbl_smoke.sh that builds cmd/mockgrbl, starts it with a unique PTY symlink and debug HTTP address, waits for the symlink and GET /state readiness, prints diagnostics on failure, and guarantees process cleanup via trap.
• Add a Linux-only integration harness package internal/integration/mockgrbl with mockgrbl_e2e_test.go (build tag linux && serial && mockgrbl_e2e) that builds/launches the mock per-test, waits on /state, returns the serial symlink and HTTP base URL, captures logs, and ensures cleanup and diagnostics.
• Add initial DDGo-over-mock integration tests that use the real serial transport and app.Controller to exercise status polling, a jog-then-stop scenario, and assert range-based machine positions rather than precise timings.
• Improve cmd/mockgrbl PTY handling by configuring the slave in raw mode (cmd/mockgrbl/pty_linux.go) and tolerate transient PTY EIO reads while preserving EOF/closed behavior (cmd/mockgrbl/main.go).

Testing

• Ran go test ./... which succeeded for the repo packages and go test -tags serial ./internal/transport ./internal/ports ./internal/app ./internal/grbl which succeeded.

• Executed the smoke script scripts/ci_mockgrbl_smoke.sh which reported readiness (mockgrbl smoke test ready).

• Executed the integration suite go test -tags 'serial mockgrbl_e2e' ./internal/integration/mockgrbl which passed locally.

• Commits included: e2fbe5a — ci: smoke test mockgrbl startup, 657106b — test: add mockgrbl integration harness, and 5887a3b — test: exercise DDGo against mockgrbl, and CI uses Ubuntu-only steps in the existing test job rather than adding a new job.

• Known remaining gaps: no UI-level integration yet, no macro/probe/contour scenarios over the mock, the mock intentionally approximates some firmware behaviors, and integration tests assert ranges (tolerances) rather than exact real-time positions.

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Codex Task