tracks: new /tracks generator for IKEA Lillabo / Brio train tracks

Wraps torwanbukaj/ikea-brio-others-compatible-train-tracks-generator
( https://github.com/torwanbukaj/... ) as a third generator alongside
holder and the universal scad playground.

Approach
--------
The upstream .scad ships as "library + one active example call". Used
include<> (rather than use<>) so the library's top-level globals
(track_width, plug/nest dimensions, $fn = 150, etc.) are available
to the modules — `use<>` does NOT propagate variables. Commented out
the upstream `track_tester();` call so include<> doesn't also emit
the tester geometry every time.

Per-render the backend builds a tiny wrapper SCAD on the fly:

    include <scad/train_tracks.scad>
    track(length=100, end1="plug", end2="nest", cutout=true, ...);

and hands it to holder.render_source().

Part types exposed (9)
----------------------
- tester        Calibration block (15-60mm)
- track         Straight (length + chamfers + grooves + plug/nest ends)
- arc           Curved (radius + angle, IKEA/Brio/J'adore radii in help text)
- dogbone       Nest-nest connector
- intersection  Crossing (angle + 2 lengths + 4 ends)
- switch        Turnout (left/right radius+angle, straight branch, common end)
- snake         S-curve (raised default target_length to 200 — the upstream
                modules's natural curvy-span at angle=45/radius=86 is ~150
                and the assert fires below that)
- adapter       BRIO <-> IKEA system adapter (plug + nest side)
- bridge        Multi-part (overview/ground/slope/pillar — value_map
                translates the UI labels to the int 0..3 the SCAD wants)

Files
-----
- scad/train_tracks.scad      Downloaded upstream (57 999 bytes), only
                              modification is the // before the top-level
                              track_tester() call.
- tracks.py                   PART_SCHEMAS dict, build_wrapper_scad,
                              render() that delegates to render_source.
- app.py                      GET /api/tracks/params, POST /api/tracks/render,
                              GET /tracks page route.
- static/tracks.html          Page with part selector + dynamic param form +
                              shared viewer markup. Reuses holder.css.
- static/js/tracks-app.js     Controller. Switching the part select redraws
                              the form (each part has its own schema).
                              Ctrl+Enter renders, Cancel uses AbortController.

Nav
---
Tracks link added to topbar on holder / index / scad.

Smoke test
----------
All 9 parts render with default params on the Manifold backend in
under 0.5s each (output sizes 440 KB - 1.8 MB).

static/scad.html

@@ -26,6 +26,7 @@
   <nav class="topbar-nav">
     <a href="/holder">Holder</a>
     <a href="/">Busbars</a>
+    <a href="/tracks">Tracks</a>
     <a href="/scad" class="active">SCAD</a>
   </nav>