Render declared fields alongside solved velocity in pyflow play (TASK-051)

[AdamClemens]

Summary

Closes Stage 8's Completion Criterion 7 -- and with it, the entire
Stage 8 reopening (see #83, #84, #87, #86): all nine criteria are now
met. pyflow play no longer rejects a config combining
simulation.velocity_solved with declared fields (Smoke Transport's
own shape), and instead renders both the flow's own arrows and each
declared field's configured panel together.

• Extracted bootstrap.py's own private panel-rendering helpers
  (_panel_colors, _panel_caption, _PanelRenderState,
  _add_panel_legend) into rendering/field_visualization.py as
  public functions, the same "extract before reusing" precedent
  TASK-045 set for simulation_run.py.
• build_panel_legend became a pure builder in the process (no
  window parameter, returns objects instead of mutating a scene
  directly) rather than moving as-is -- the original shape would have
  violated field_visualization.py's own "owns no window" rule.
• The extraction was verified behaviour-preserving by the full
  pre-existing test suite most likely to be affected (117 tests across
  test_bootstrap.py, test_field_visualization.py, every
  tests/golden/ module) passing unmodified, before anything new
  was added.
• playback.py's combined path renders both arrows and panels
  together -- a real, deliberate widening beyond what bootstrap.py's
  own live pyflow run path currently does for the identical config
  shape (that path draws panels but never arrows alongside them, a
  pre-existing, separately flagged gap -- not fixed here, out of
  scope).

Test plan

• Strict TDD throughout
• Behaviour-preservation verified for the extraction before adding
  anything new
• make lint and every relevant make check-* target clean
• make test -- 1178 passed, 21 skipped, exit 0
• Verified by hand against the real CLI and the real rendered
  scene: a Smoke Transport record-then-play round trip produced
  259,649 non-background pixels across 1,719 distinct colours, with
  window.scene holding exactly the expected shapes -- 2 legends
  (128 vertices each), 2 field panel meshes (1,024 vertices each),
  the 2-vertex scrub-bar track, and a 1,536-vertex arrow object

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