Plug-ins: compiling a plug-in's code takes none of a run's instructions

wasmi compiles each function of a plug-in when it is first called and,
by default, charges the run that calls it 7 instructions for each byte
of the function. The host hands a run its budget in 20 M-instruction
slices and resumes it when a slice runs out. But when a slice runs out
while a function compiles, wasmi returns an error from the call, not an
out-of-fuel pause, and the host reports it as Limit("instruction
budget"). So a plug-in's first run could fail with most of its budget
left, depending on where the slices fell, and the next run, with its
functions compiled, succeeded.

StitchCraft's PES exporter hit it on a design of 98 × 148 mm. In
VectorCraft 0.17.0's host its first export failed with 10 G and with
275 G instructions alike, and its second succeeded, taking under 1 G.
Run on wasmi directly, with the host's slices, the first export failed
in its 36th slice ("required_fuel=174139"), and with compiling free it
finished in 49.

Compiling now costs nothing: wasmi's fuel costs with no cost for
translating or validating, and its default cost for copying. A run pays
for its own instructions only, as many on its first run as on the next.
Compiling every function when the plug-in loads (CompilationMode::Eager)
also fixes it, but loads a 1.4 MB module in 42 ms instead of 10.

The new test fails without the change: a run that takes 1,040
instructions once its functions are compiled failed within them on a
plug-in just loaded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhvnnnD5fTFupK8WWq35un

docs/plugins.md

@@ -109,6 +109,11 @@ wasmi is built with its *portable* (loop) dispatcher: the default tail-call disp
 sibling-call optimisation, and when that doesn't happen a long-running module overflows the host stack, which aborts
 the process instead of returning an error.
 
+wasmi compiles each function when it is first called, and the instruction budget doesn't pay for that, so a run
+takes as many instructions the first time as the next. Charged to the run, as wasmi charges by default, compiling
+could fail it: when the 20 M-instruction slice the run was in ran out while a function compiled, wasmi ended the run
+with an error rather than a pause the host resumes, and the run stopped with most of its budget left.
+
 Drawing evaluates effects often, so live-effect results are cached by plug-in installation, parameters and input
 geometry: a plug-in runs again only when one of them changes.