continue. FIX ROUND 6 (start it right after round 5 is rendered and reported; if round 5 is still running, finish it first). Design already decided, just implement. Keep your reasoning short: one edit, run the measuring tool, look at crops, next edit.

THE PROBLEM (Kavan circled it on a screenshot of the engine head): the cam lobes, cam towers, bearing rings, rollers (lifters), valve barrels / stems / heads and the head casting are all the SAME steel grey, so where they touch they merge into one blob. The springs are dark on a dark band and dark on grey, the plug is dark on a near-black background. His words: lifters, valves and springs should be outlined; maybe everything should be outlined; dark things get a light outline, light things get a dark outline; or change the colours of the cams and lifters slightly so you can tell the parts apart. Do BOTH: outlines and slight tints.

THE DESIGN (implement exactly this; index.html, engine group around lines 36-150):

A. LIGHT parts (everything filled url(#steelGrad), i.e. head block, cam towers, crankcase pan, tubes and outer disc, crank disc and crank pin disc, piston body, rod bosses, valve heads / stems / barrels, cam lobes, rollers): dark outline stroke #0E1116, stroke-width 2, stroke-linejoin round. The rod polygon, rodSmall and the piston already have dark outlines on some parts: make all consistent. Easiest: one CSS rule block keyed on fill (for example [fill="url(#steelGrad)"] { stroke:#0E1116; stroke-width:2; stroke-linejoin:round }) plus the same for any element you give a new flat fill. Note CSS beats presentation attributes. Do NOT add an outline to the big black cavity rects (fill #0E1116) or to the hatch bands.

B. DARK parts (fill #2B3038: both plug rects, valve retainers and washers, wrist pin, pin boss, the 6 piston ring-groove rects, the two cam bearing holes r12, the counterweight, the dimmed flywheel disc): light outline stroke #AEB8C2, stroke-width 1.5 (same CSS approach, key on fill="#2B3038").

C. SPRINGS: the dark coil needs a light edge. Draw an underlay polyline for each spring (springIntakeUnder, springExhaustUnder) with stroke #C9D1D9, stroke-width 9, stroke-linejoin round, fill none, using exactly the same points as the coil, placed just BELOW the dark coil in z-order; set the dark coil to stroke-width 5. Both polylines must be updated every frame with the same points. The spring must still be in front of the head casting.

D. BEARING RINGS (the two r20 stroke rings at (856,204) and (944,204)): add an underlay circle at the same centre, r20, fill none, stroke #0E1116, stroke-width 10, just below each ring, so the ring has a dark edge on both sides.

E. SLIGHT TINTS (so the parts differ even before you look at the outlines). Change only the fills of these, nothing else:
   - cam lobes (camIntake, camExhaust): #B7A88E (a muted tan)
   - rollers (rollerIntake, rollerExhaust, the lifters): #D8CEBA (a lighter tan)
   - valve stems, barrels and heads (valveStem*, valveBarrel*, valveHead*): #C5CDD5 (a lighter cool grey than the casting)
   Everything else keeps the steel gradient. Do NOT use blue, yellow, orange or red on any metal: those hues mean gas in the legend. These tans are muted on purpose.

F. Do not touch the callout leaders, the dial, the captions, the HUD, the zoom or the animation. Outline widths may scale with the 1.0 to 1.2 zoom; that is fine.

CHECK: node measure_engine.mjs --times 5.5,7.33,8,11.5,16,21.5,24,27.5,31 --shots workspace/meas6 . All 17 checks must pass. If a check fails ONLY because it samples a colour you changed on purpose (for example a contrast or spring-visible test), adapt that check in measure_engine.mjs minimally, and say exactly what you changed and why in REPORT.md. Do not loosen any check that a real defect would trip.
Then LOOK (ffmpeg -ss T from the new final.mp4; head crop = crop=330:260:760:150 then scale 4x; lower crop = crop=500:640:650:370 then scale 2x) at t=2.0, 5.5, 11.5, 24 (head) and t=2.0, 9.7 (lower). For each of these pairs, state in REPORT.md what visibly separates them: cam lobe | cam tower, roller | cam lobe, valve barrel | stem | head, retainer | spring, spring | head casting, plug | background, piston | cylinder wall, rod | crank disc. Every pair needs a visible edge or a visible tint step. If any pair still merges, fix it and look again.

THEN: BEFORE rendering back up the round-5 renders: final.mp4 -> final_v5.mp4 and review.mp4 -> review_v5.mp4 (do not overwrite older backups). Render review.mp4 and final.mp4; ffprobe gate (h264, 1 video stream, 1920x1080, 30/1, 32.0 s +/-0.05, no audio); append round 6 to REVIEW.md, REPORT.md, NOTES.md. Final report: per item done or not done with the evidence, the gate, files written; be honest about anything still imperfect.