ci: dual-lane simple-math smoke (no-think + thinking) — from #67

[antmikinka]

Why

Mainline vehicle for stacked #67 dual-lane CI after product stacks landed:

Stack	Mainline PR
#64 ChatSession / EOS	#70
#65 stop / thinking floor	#71
#66 load hygiene	#72

There was no prior merge/pr67-into-main. Stacked #67 still targets obsolete split/pr63-03-load-hygiene.

What

CI-only (3 files):

Area	Change
build-mlx-engine.yml	Dual-lane matrix: no-think + thinking × ubuntu-rocm / ubuntu-cpu / macos-arm64
budgets	ROCm thinking max_tokens=4096; CPU/mac thinking 512 (prior OOM/timeout class)
other smokes	enable_thinking: false in action.yml + test-mlx-engine.yml
benches	Mode-scoped artifacts bench-results-${{ matrix.os }}-${{ matrix.mode }}

Explicitly NOT in this PR

• No C++ / thinking_budget.h / server floor policy changes
• Does not re-land stacked raise-low floor (main keeps fix(server): stop sequences, thinking floor, role:tool 400 (from #65) #71 nullopt-only)
• No HTTP floor-probe hard gate (omit max_tokens cannot hit nullopt — ChatCompletionRequest::max_tokens defaults to 4096). Floor covered by tests/test_thinking_budget.cpp

Supersedes

Stacked #67 (split/pr63-04-ci-dual-smoke → split/pr63-03-load-hygiene). Close #67 in favor of this PR when green.

Merge policy

• Human merge only when checks are green
• Do not self-merge
• Addresses Geramy on split(#63)/4: CI dual-lane smoke (no-think + thinking) #67: dual-lane on main + CI reliability adaptations