refactor: propagate ctx into the four public constructors

[robbyt]

Summary

Finishes the work started in PR #144. The four public consumer constructors now take ctx as the first argument:

• polyscript.New[E](ctx, src, opts...)
• extism.FromExtismLoader(ctx, ldr, opts...)
• risor.FromRisorLoader(ctx, ldr, opts...)
• starlark.FromStarlarkLoader(ctx, ldr, opts...)

The three inline context.Background() injections in engines/{extism,risor,starlark}/new.go are gone. Cancellation now actually reaches the loader's I/O and the compiler's parse path.

deprecated.go

The twelve legacy FromXxx* constructors keep their signatures unchanged and inject context.Background() inline at each downstream call to From*Loader. They are slated for removal in #104; churning their signatures during the deprecation window adds caller pain without value.

New tests

• engines/risor/new_test.go::TestFromRisorLoader_CompileCancelled — pre-cancelled ctx aborts the Risor parser. Asserts errors.Is(err, context.Canceled).
• engines/extism/new_test.go::TestFromExtismLoader_LoaderCancelled — pre-cancelled ctx aborts the loader's GetReader. (Wazero's compile of the small embedded test module completes too fast to reliably demonstrate parser-level cancellation; the loader-IO cancellation path is the meaningful one for end-users anyway.)
• polyscript_test.go::TestNew_CompileCancelled — top-level Risor cancellation via polyscript.New[polyscript.Risor].
• polyscript_test.go::TestNew_HTTPLoaderCancelled — end-to-end demonstration: a stalling httptest.Server, a 50ms ctx deadline, an assertion that the fetch aborts near the deadline (rather than waiting for the 2s server stall) and returns context.DeadlineExceeded. This is the precise scenario [v1][breaking] Propagate ctx into polyscript.New[E] and the three FromXxxLoader constructors #145 set out to fix.

Files

• Modify: polyscript.go, engines/{extism,risor,starlark}/new.go — add ctx as first arg
• Modify: deprecated.go — inject context.Background() inline at each call
• Modify: all 9 examples/*/main.go — thread ctx
• Modify: ~8 _test.go files — pass t.Context() at call sites
• Add: 4 new cancellation tests (per-engine + top-level + HTTP end-to-end)
• Modify: CHANGELOG.md — [Unreleased] > Changed

Net diff: 23 files changed, 258 insertions, 120 deletions.

Closes #145.

Test plan

• go build ./... clean
• go vet ./... clean
• go test -race -count=1 ./... full suite green
• grep -rn 'context\.Background()' --include='*.go' engines/{extism,risor,starlark}/new.go polyscript.go returns zero hits
• grep -c 'context\.Background()' deprecated.go returns 12 (one per legacy constructor)
• HTTP loader cancellation: stalling server aborts near the ctx deadline (~50ms) rather than waiting for the full 2s server stall
• CI: SonarQube, dependency-review, Copilot review

https://claude.ai/code/session_01C61VEAmjxSnX5Xhbab8NvL

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Generated by Claude Code

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR completes the context-threading refactor by adding context.Context as the first parameter to the four public consumer constructors so cancellation can reach loader I/O (e.g., HTTP fetch) and engine compile/parse paths.

Changes:

• Add ctx context.Context as the first argument to polyscript.New[E] and the three engine-level From*Loader constructors.
• Remove inline context.Background() injection from engine constructors; keep deprecated constructors unchanged by injecting context.Background() at call sites.
• Add/adjust tests and examples to pass t.Context() / context.Background() and add cancellation-focused test coverage.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
readme_test.go	Updates README-backed tests to pass t.Context() into polyscript.New.
polyscript.go	Changes New[E] signature to accept ctx and threads it into engine construction.
polyscript_test.go	Updates call sites and adds end-to-end cancellation tests (compile + HTTP loader).
polyscript_options_test.go	Updates option/constructor tests to pass t.Context().
polyscript_extism_test.go	Updates Extism construction tests to pass t.Context().
examples/simple/{risor,starlark,extism}/main.go	Threads a context into polyscript.New in examples.
examples/multiple-instantiation/{risor,starlark,extism}/main.go	Threads a context into polyscript.New in examples.
examples/data-prep/{risor,starlark,extism}/main.go	Threads a context into polyscript.New in examples.
engines/starlark/new.go	Adds ctx to FromStarlarkLoader and forwards it into script.NewExecutableUnit.
engines/starlark/new_test.go	Updates Starlark engine constructor tests to pass t.Context().
engines/risor/new.go	Adds ctx to FromRisorLoader and forwards it into script.NewExecutableUnit.
engines/risor/new_test.go	Updates Risor engine constructor tests and adds cancellation test.
engines/extism/new.go	Adds ctx to FromExtismLoader and forwards it into script.NewExecutableUnit.
engines/extism/new_test.go	Updates Extism engine constructor tests and adds loader-cancellation test.
engines/integration_test.go	Updates integration tests to pass t.Context() into engine constructors.
deprecated.go	Keeps deprecated constructors’ signatures and injects context.Background() at downstream calls.
CHANGELOG.md	Documents the breaking change and the new cancellation behavior.

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