feat: add builtin runtime support

[abdulrahman11a]

Summary

Closes #4307

This PR implements a built-in runtime for curated KRM functions inside kpt itself,
allowing apply-replacements and starlark to run without pulling images from Docker Hub.

Approach

As discussed in the issue thread, this implementation avoids reintroducing a circular
dependency between Porch and kpt by moving the built-in runtime concept into kpt directly.
The architecture is intentionally simple and extensible — new functions can be added
incrementally by registering them in the builtin registry.

How it works

A thread-safe self-registration registry (internal/builtins/registry) allows KRM function
implementations to register themselves via init(). The fnruntime runner checks this
registry before falling back to Docker or WASM, so existing behavior is fully preserved
for unregistered functions.

Priority order in fnruntime/runner.go:

1. pkg-context builtin (existing)
2. Builtin registry ← new, no Docker needed
3. Docker / WASM (fallback, unchanged)

Functions included

Function	Image
apply-replacements	ghcr.io/kptdev/krm-functions-catalog/apply-replacements
starlark	ghcr.io/kptdev/krm-functions-catalog/starlark

Files changed

File	Description
internal/builtins/registry/registry.go	Thread-safe self-registration registry
internal/builtins/applyreplacements/apply_replacements.go	Built-in apply-replacements implementation
internal/builtins/starlark/starlark.go	Built-in starlark entry point
internal/builtins/starlark/config.go	Starlark config parser (accepts StarlarkRun and Run kinds)
internal/builtins/starlark/processor.go	Starlark processor
internal/builtins/BuiltinRuntime.go	Blank-imports all builtins to trigger init()
internal/fnruntime/runner.go	Wires builtin registry lookup before Docker fallback

Verified locally

[PASS] "ghcr.io/kptdev/krm-functions-catalog/apply-replacements:v0.1.1" in 0s
[PASS] "ghcr.io/kptdev/krm-functions-catalog/starlark:v0.5.0" in 0s

Both functions execute without Docker running.

Next steps

• More curated functions can be gradually inlined following the same pattern
• WASM support remains a separate investigation track as suggested in the issue

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[Copilot]

Pull request overview

This PR implements a built-in runtime for curated KRM functions (apply-replacements and starlark) inside kpt itself, allowing them to run without Docker by registering them in a thread-safe in-process registry. When a function image matches a registered builtin (by normalized name, ignoring tags/digests), the builtin is used; otherwise the existing Docker/WASM fallback is preserved.

Changes:

• New self-registration registry (internal/builtins/registry/) with thread-safe Register/Lookup using image name normalization
• Built-in implementations for apply-replacements and starlark functions, wired via init() blank imports
• Integration into fnruntime/runner.go to check the builtin registry before falling back to Docker/WASM

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
internal/builtins/registry/registry.go	Thread-safe builtin function registry with image name normalization
internal/builtins/applyreplacements/apply_replacements.go	Built-in apply-replacements KRM function implementation
internal/builtins/starlark/starlark.go	Built-in starlark KRM function entry point
internal/builtins/starlark/config.go	Starlark config parser supporting StarlarkRun, Run, and ConfigMap kinds
internal/builtins/starlark/processor.go	Starlark processing logic
internal/builtins/starlark/config_test.go	Tests for starlark config parsing
internal/builtins/BuiltinRuntime.go	Blank-imports all builtins to trigger init() registration
internal/fnruntime/runner.go	Wires builtin registry lookup between pkg-context and Docker/WASM
go.mod / go.sum	Adds starlark catalog dependency and upgrades fn SDK to v1.0.2

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

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