re: clear mirror handling and the exhaust cones -- the viewer bug's static avenue is exhausted

Mirror handling: the concern was that apply() bakes R*(S*v)+T while the viewer
decides winding from det(m) alone and rotates normals by m alone, so a mirror
encoded as negative SCALE would reflect geometry without flipping winding. It
never happens -- across 1485 assembled parts in all 22 containers there are 22
mirrored parts, every one with det(m) < 0, and ZERO negative or non-uniform
scales. apply_twin_mirrors writes the reflection into m, so the flip always
fires and ignoring s for normals is harmless.

Exhaust cones: the only geometry the viewer invents, drawn at GN_Jet/GN_SJet
frames because the real engine geometry is recessed. Across 335 assembled ships,
192 with exhaust frames, not one cone sits outside its hull bounding box (10% of
span tolerance). Caveat kept in the doc: inside-the-box does not prove a cone is
correctly oriented or sized -- it rules out the reported symptom, no more.

Every mechanism the diagnosis proposed is now eliminated (include_external,
duplicate names, mirrors, cone placement). The visual comparison is no longer
the next step but the only remaining one, and if it agrees the original report
needs re-grounding against a specific ship and expectation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/re/BACKLOG.md

@@ -113,3 +113,40 @@ resolves to the one model it names.
 **So two of the three candidates are gone** (this one and `include_external`),
 leaving **mirror handling** and **the exhaust cones** — and the still-untried
 visual comparison, which remains the right next step.
+
+---
+
+## Viewer: mirror handling and the exhaust cones are cleared too — the static avenue is exhausted
+
+**Checked 2026-08-11.** Both remaining candidates were tested across every ship
+on the disc, and neither shows the reported signature.
+
+**Mirror handling.** The concern was that `ScenePart::apply` bakes `R·(S·v)+T`
+while the viewer takes its winding-flip decision from `det(m)` alone and rotates
+normals by `m` alone — both ignoring `s`. A mirror encoded as a *negative scale*
+would then reflect geometry without flipping winding, drawing the part
+inside-out. It never happens: across **1 485 assembled parts** in all 22
+containers there are **22 mirrored parts, every one with `det(m) < 0`**, and
+**zero** parts with a negative scale or a non-uniform one. `apply_twin_mirrors`
+writes the reflection into `m` (negating its X column), so the viewer's flip
+always fires, and ignoring `s` for normals is harmless because `s` is always
+uniform.
+
+**Exhaust cones.** These are the one piece of geometry the viewer *invents* — a
+cone at each `GN_Jet`/`GN_SJet` frame, because the real engine geometry is
+recessed and the game draws FX there instead. If they landed wrongly they would
+read exactly as "a part in the wrong place". Across **335 assembled ships, 192 of
+which have exhaust frames, not one cone sits outside its hull's bounding box**
+(tolerance 10 % of the axis span).
+
+**Caveat, stated rather than glossed:** "inside the hull box" does not prove a
+cone is *right* — orientation and size are untested, and a cone could be wrong
+while still inside. What it does rule out is the reported symptom for that part.
+
+So every mechanism this diagnosis proposed is now eliminated: `include_external`,
+duplicate resource names, mirror handling, and cones-in-the-wrong-place. The
+format and assembly layers pass every static test available, and **the visual
+comparison is no longer merely the next step — it is the only remaining one.**
+Render `e106` in the viewer beside `ship_render`'s output of the same
+`assemble_ship` result; if they agree, the bug is in neither and the original
+report needs re-grounding against a specific ship and a specific expectation.