Add FlowReactor/FlowReactorSurface support (distance-marched PFR)

[erwanp]

Summary

Implements the deferred design doc docs/plans/2026-07-14-flow-reactor-support.md (removed by this PR now that it's built): support for Cantera's ct.FlowReactor (plug-flow, distance-marched) and an attached ct.ReactorSurface (catalytic wall chemistry), so scripts like upstream Cantera's surf_pfr.py can be modeled in Boulder end-to-end, both directions (YAML → Cantera and Cantera → YAML).

• solver.axis: distance (boulder/config.py) — new opt-in stage-level solver key, default "time". Validated in the single choke point (_resolve_and_validate_solver_mode) shared by both STONE dialects (network: and stages:), so it never changes default behavior for existing time-integrated reactors.
• FlowReactor node (boulder/cantera_converter.py) — new branch in create_reactor_from_node, following the existing per-reactor-type branch style: area, mass_flow_rate, surface_area_to_volume_ratio, energy.
• Surface chemistry — modeled as a surface: property on the FlowReactor node rather than a new connection kind. ct.ReactorSurface isn't a flow-graph edge the way ct.Wall is, so a connection-based design would have forced it into stage-DAG/Sankey semantics that don't apply. The builder constructs the surface Interface first and derives the reactor's gas phase from its .adjacent[...], mirroring Cantera's own surf_pfr.py pattern (required for kinetics consistency between the two phases).
• Distance-axis plotting — required no frontend code changes. The existing is_spatial/x series convention (built for CSTR-chain PFR approximations) already renders "Position (m)" plots in PlotsTab.tsx; the transient-solve trajectory recorder now tags distance-marched series with is_spatial: true instead of is_residence: true, reusing that path.
• sim2stone reverse direction (boulder/sim2stone.py) — detects FlowReactor/attached surfaces directly off the live Cantera object (no AST guessing needed, unlike Func1 schedules, since nothing here is a discarded parametric form). One binding quirk found along the way: FlowReactor.mass_flow_rate is write-only in the Cantera 3.2 Python API — recovered via continuity (density × speed × area) instead of a direct read.
• Example: configs/surf_pfr.yaml + docs/cantera_examples/surf_pfr.py (methane partial oxidation over Pt, vendored from upstream Cantera), wired into the existing fixture-script test harness (tests/test_sim2stone/test_fixture_scripts_sim2stone.py).
• Tests: tests/test_sim2stone_flow_reactor.py covers both directions.

Known gap

download_script_emitter.py's standalone-script codegen doesn't yet support FlowReactor/FlowReactorSurface — it's a separate, hardcoded reactor-dispatch pathway from create_reactor_from_node. The two --download tests for surf_pfr.py are marked xfail for this reason; YAML emission, --output-yaml, and validate all work correctly.

Note on scope vs. the original design doc

The plan referenced a boulder_examples/examples/manifest.yaml catalog with status: unsupported entries — that catalog doesn't exist in this repo (checked: not in git history, not a submodule). The example/test targets were adapted to this repo's actual conventions (docs/cantera_examples/ + configs/ + tests/test_sim2stone/).

Test plan

• make type-check (mypy strict) — clean
• make qa (pre-commit run --all-files) — clean
• Full backend suite: 694 passed, 10 skipped, 7 xfailed, 0 failed
• Manual end-to-end smoke test: STONE YAML → DualCanteraConverter → real catalytic chemistry (CH4 consumed, CO produced) along the distance profile
• Manual round-trip smoke test: hand-written ct.FlowReactor + ct.ReactorSurface script → sim_to_stone_yaml → re-parsed and rebuilt successfully
• No frontend changes were made, so no npm run build/npm run test:unit was needed for this PR

Extensive AI use — code & PR fully written by Claude Sonnet 5

🤖 Generated with Claude Code

Added as an example in Boulder-Examples

parks4/boulder_examples#17

• Yields the same H2 mole fractoin output (~0.25%) as the Cantera base example https://cantera.org/dev/examples/python/reactors/surf_pfr.html