holder: fix decimal -D overrides being truncated to int

_filter_params() had a stale short-circuit:

    v = float(v) if p.step and p.step != int(p.step) else int(float(v))

For auto-extracted params (the new majority of PARAMS), `p.step is None`,
which is falsy. The conditional took the `int(float(v))` branch and
truncated values like 0.2 -> 0, 0.7 -> 0, 1.5 -> 1. The /holder render
silently dropped every clearance, tolerance, and offset to zero, so
the holder pieces in "both" mode (and a handful of other configurations)
came out merged at their edges. /scad never hit the bug because it
doesn't pass -D overrides — the SCAD defaults applied unchanged.

Fix: always coerce through float, then narrow back to int only when
the result has no fractional part. Verified /holder render with all
defaults now produces the exact same STL bytes as /scad with the
bundled source (md5 match).

holder.py

@@ -305,10 +305,11 @@ def _filter_params(params: dict) -> dict:
             continue
         p = known[k]
         if p.kind == "number":
-            v = float(v) if p.step and p.step != int(p.step) else int(float(v))
+            # Always coerce through float — earlier logic short-circuited to
-            # keep ints for integer-step params
+            # int(float(v)) when step was None (most auto-extracted params),
-            if isinstance(p.default, int) and float(v).is_integer():
+            # truncating decimals like box_clearance=0.2 → 0.
-                v = int(v)
+            fv = float(v)
+            v = int(fv) if fv.is_integer() else fv
         elif p.kind == "bool":
             v = bool(v)
         elif p.kind == "select":