Preserve source cadence and sample picture fps after analysis

docs/port-plan.md

@@ -4,8 +4,10 @@ Self-contained. You should not need any prior conversation to execute this.
 
 **Implementation status (2026-09-27):** steps 0–6 are in the CLJS frontend.
 Step 6 reads extracted footage from the manifest, detects landmarks with local
-MediaPipe assets, and runs the same freeze path as the synthetic take. Step 7 is
-next: eyes, brows and the pixel-derived mouth interior.
+MediaPipe assets at full source cadence, and runs the same freeze path as the
+synthetic take. The scene time map can sample the frozen roto at a lower picture
+fps without changing source analysis, duration or audio. Step 7 is next: eyes,
+brows and the pixel-derived mouth interior.
 
 ## What arthur is
 
@@ -302,6 +304,10 @@ MediaPipe interop behind one namespace; real frames, real audio, real fps from
 the manifest. **Vendor the wasm** rather than fetching from jsdelivr — it is
 currently the only thing in the tool that silently requires a network.
 
+Decode every source frame for analysis. A lower output picture fps is a time map
+over frozen channels, not a reduced detection track. Selecting source frames to
+trace into cels is independent again and remains outside this port's paint scope.
+
 **Done:** real footage plays back as a rotoscoped mouth.
 
 ### 7 — the rest of measure