ci(boards): add LPC845-BRK + LPCXpresso804 stub workflows and README badges

[zackees]

Summary

• Adds two no-op stub CI workflows — build-lpc845brk.yml and build-lpcxpresso804.yml — plus three README badges (LPC845-BRK, LPCXpresso804, LPCXpresso845-MAX; the last workflow already exists on main via [codex] add ArduinoCore LPC8xx board metadata #514).
• Part of the board-coverage effort in lpc8xx: convert 5 stub workflows to real builds (Stage 8 of #487) — orchestrator + upstream proof are in #528 (Stage 8 of META [META] LPC8xx end-to-end support: orchestrator + Arduino framework + real FastLED sketches #487): every protected board gets a named CI slot now; the stubs get swapped for real template_build.yml callers once in-tree fixtures land.

Why stubs, not real builds

Quoting the in-file rationale:

No-op stub. The NxpLpc orchestrator DOES build this variant — proof: fbuild build . -e lpc845brk is green on main in upstream zackees/ArduinoCore-LPC8xx CI (variant source of truth). This stub only exists because there's no in-tree fixture for it yet (no tests/platform/lpc845brk/; the board JSON boards/json/lpc845brk.json landed in PR #514).

History: a previous attempt used continue-on-error: true on the reusable-workflow caller, but that keyword is NOT in GitHub's allow-list for reusable callers — every push went red with a "workflow file issue" before any job actually ran. The no-op pass below sidesteps that limitation.

What was deliberately dropped from the original draft

The draft also contained a stub replacement for build-lpcxpresso845max.yml. That premise is stale: PR #514 (merged 2026-06-09) already shipped the real template_build.yml caller together with tests/platform/lpcxpresso845max/platformio.ini and boards/json/lpcxpresso845max.json, and the workflow has been green on main for its last 10 runs. Stubbing it out would regress a working build, so this PR leaves it untouched.

Test plan

• actionlint clean on both new workflow files
• Stub jobs only run echo steps — expected green
• Full check matrix green (or failures pre-existing on main)

Refs #528, #487

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ℹ️ Review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between 2be62bf and 3420234.

📒 Files selected for processing (3)

• .github/workflows/build-lpc845brk.yml
• .github/workflows/build-lpcxpresso804.yml
• README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/lpc8xx-stub-workflows

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