SILE with Lua 5.1 issue with "before kern" styling

[Omikhleia]

Observed

$ sile examples/lefevre-tuor-idril.sil 
SILE v0.15.5 (LuaJIT 2.1.1723675123) [Rust]
<examples/lefevre-tuor-idril.sil> as sil
</usr/share/sile/core/sile.lua:622> as lua
[1] [2] Error: runtime error: ./packages/resilient/styles/init.lua:575: attempt to compare table with number
stack traceback:
	[C]: in function '__lt'
	./packages/resilient/styles/init.lua:575: in function <./packages/resilient/styles/init.lua:544>
(...)

Fix

In packages/resilient/styles/init.lua :

   SILE.registerUnit("thsp", {
     relative = true,
     definition = function (value)
-      return value * 0.5 * utils.interwordSpace()
+      return value * 0.5 * utils.interwordSpace():tonumber()
     end
   })

Analysis

Overlooked difference between Lua 5.4 (implicit conversion applies) and Lua 5.1.

[Omikhleia]

Clearly overlooked: my older SILE 0.14 with Lua 5.1 (actually LuaJIT) build from #70 also has the issue. (Just to clarify it's not related to the recent SILE 0.15 updates -- I had forgottten this "example" article)