Update default yaml examples

[erwanp]

• Replace configs/default.yaml with a lean methane/air CSTR combustor (φ = 0.5), complete inlet/outlet topology, and a non-empty output: block aligned with the GUI default load path.
• Add configs/cstr_methane_air.yaml as an annotated sweep-ready variant and standardise output: across the curated configs/*.yaml set.
• Extend CI coverage: default config existence/non-empty output, full curated-library checks, and BoulderRunner.solve() on default.yaml.
• Fix STONE round-trip export so optional connection/node fields are not re-emitted when unset (mechanism_switch, metadata).
• Fix duplicate operating pressure on export for IdealGasConstPressureMoleReactor when initial.pressure is already set; register that kind in const-pressure handling used by the exporter.
• Refresh configs/README.md for the real file list and physical intent of each example.
• Add warn_simulation_quality notes on boulder validate when output: is missing or empty (non-fatal).