[v1][breaking] Propagate ctx into polyscript.New[E] and the three FromXxxLoader constructors

[robbyt]

Summary

Follow-up to #89 (PR #144). #89 added ctx context.Context as the first arg to Compiler.Compile, Loader.GetReader, and script.NewExecutableUnit, but stopped one layer short of the public consumer surface. The three FromXxxLoader constructors and the top-level polyscript.New[E] still don't accept a ctx, so they inject context.Background() inline when calling NewExecutableUnit:

// engines/extism/new.go:102
execUnit, err := script.NewExecutableUnit(context.Background(), cfg.handler, ...)

Same pattern in engines/risor/new.go and engines/starlark/new.go. The ctx-threaded path through Loader.GetReader(ctx) and Compiler.Compile(ctx, reader) ends up exercising a never-cancellable Background() for every caller using the documented public API.

What this means in practice

The HTTP-loaded compile problem #89 set out to solve — "compile can hang indefinitely because nothing reaches for the cancellable variant" — is not actually fixed for the public consumer surface. We moved the context.Background() injection point from inside the loader to the caller side of NewExecutableUnit, but nobody using polyscript.New[E] or FromExtismLoader/FromRisorLoader/FromStarlarkLoader can supply a cancellable ctx today.

Fixes 1 and 2 from #89 (Extism's stored-ctx field, Risor's hard-coded Background() in the inner parser) are real and complete. Fix 3 (HTTP cancellation) is only reachable by callers who drive script.NewExecutableUnit directly — a building-block API that the docs steer people away from.

Proposal

Add ctx context.Context as the first arg to:

• polyscript.New[E](ctx, source, opts...) — top-level generic constructor
• extism.FromExtismLoader(ctx, ldr, opts...)
• risor.FromRisorLoader(ctx, ldr, opts...)
• starlark.FromStarlarkLoader(ctx, ldr, opts...)

Each forwards its ctx straight into script.NewExecutableUnit. The three context.Background() injections in the engine new.go files go away.

Files

• polyscript.go — New[E] signature
• engines/{extism,risor,starlark}/new.go — FromXxxLoader signatures + drop the inline context.Background()
• deprecated.go — the legacy FromRisor*/FromStarlark*/FromExtism* constructors (decide: also take ctx, or leave on the Background() shim until [v1][breaking] Remove deprecated top-level constructors (deprecated.go) #104 removes them)
• Every example under examples/ — pass ctx
• All consumer tests — pass t.Context() or equivalent

Acceptance

• grep -rn 'context\.Background()' --include='*.go' . finds no production hits in the public construction path (the inline injections in engines/{extism,risor,starlark}/new.go are gone)
• A user can supply ctx, cancel := context.WithTimeout(...) to any public constructor and have the cancel actually reach Loader.GetReader/Compiler.Compile
• Spot-check: cancelling a ctx during an HTTP-loaded compile actually halts the fetch

Notes

• This is a v1 breaking change to the four most-used public constructors.
• Was deferred from [v1][breaking] Compiler.Compile and Loader.GetReader should accept a context #89 to keep that PR's scope to the interface-level signature changes.
• Discussed in #144 review thread.