Port native grind trajectory candidate and admission leaves

docs/grind-native-assist.md

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+# Native grind assist port status
+
+The live game still supports the paired-truck50-50 path only. The new
+`skate-core/src/air/trajectory/grind.rs` leaves are not connected to gameplay.
+They must not be presented as working magnetic assistance or all-grind support.
+
+Reference: TU3 image SHA256
+f4aa113eb541bfba03dbc108cf5ab43f58c965b20fa3b82f9c40938a0ad841c4,
+generated PPC in the local Skate3Research/Skate3CustomEngineLayer checkout.
+No reference process or gameplay tests were run for this port.
+
+## Ported leaves
+
+- 82D60C80/82D60B98: quadratic trajectory-plane intersection and later-root selection.
+- 82D6A398: projected rail trajectory candidate, signed endpoint interval,
+  radius plus GrindOffset plane, miss distance, crossing time, fixed60Hz frame,
+  rail direction and approach direction.
+- 8296EC98/82E09C80: oriented approach angle and fractional-turn fold.
+- 82D6A168: height/angle ranking inside the difficulty lock distance and
+  distance fallback outside it; removing one winner preserves retry order.
+- Admission/displacement portion of82D6A840: relative height, vertical rail,
+  perpendicular speed, ledge-side lock scalars, deck-tip allowance and maximum
+  trajectory angle. It requires a supplied native surface result; no dummy
+  classification or unconditional snap is provided.
+
+These are static source translations, not a claim of verified bitwise parity.
+The core crate builds. The user has reserved gameplay testing for themselves.
+
+## Missing live dependencies
+
+1. Preserve the complete indexed world-query result set for82C20728/82C20C08,
+   then implement the rail/edge/ledge classification82C21930. The existing
+   ground hang-up adapter only produces the fields that its ground caller reads.
+2. Complete82D6A840's accepted-target publication and82D6AF58 landing-normal
+   construction. Feed the real result through trajectory scoring/second-pass
+   selection instead of the current WorldWithoutGrindEdges marker.
+3. Connect GrindAirAdjust82D712E0 and its projection, target, and correction
+   owners to the physical skeleton/board. The displacement leaf by itself is
+   not the complete airborne assist.
+4. Port the other contact probes/scorers and state owners. Native manager
+   82D875A8 uses separate centre-deck, inverted-deck, truck and tip probes.
+   State400 boardslide,40150-50,402 tipslide,4035-0,404 backslash and405
+   darkslide are distinct physics states. Naming variants requires the actual
+   chromosome producer as well as those state IDs.
+
+## Proposed read-only reference trace
+
+Awaiting user authorization because earlier instructions prohibit launching
+the recomp. No synthetic gameplay input is proposed; the user would approach
+and land on a rail in the reference game.
+
+Capture query inputs and indexed hit validity/fraction/position/normal/surface
+at82C20728 and82C20C08, the classification at82C21930, ranked candidates and
+accepted/rejected attempts at82D6A168/82D6A840, and GrindAirAdjust output at
+82D712E0. Include state/category, mode, simulation tick, spline ID, board frame
+and controller inputs so these observations can be matched to their callers.
+Keep the trace local and do not modify the reference binaries or database.