Mix the sauce and prep everything: stir the light soy sauce, dark soy sauce, kecap manis, fish sauce (if using), white pepper, and water together in a small bowl. Have the shrimp, sausage, garlic, eggs, sprouts, and chives measured and within reach before the pan heats up.
Sear the shrimp: heat a wok or large heavy skillet over high heat until it just starts to smoke. Add 1 tablespoon of the oil and swirl. Add the shrimp in a single layer and sear for 1 minute per side, just until pink at the edges. Move them to a plate.
Render the sausage and build the base: add another tablespoon of oil. Add the sliced Chinese sausage and stir-fry for 1 minute until the edges curl and some sweet fat renders. Add the garlic and sambal or chilies and stir for 20 to 30 seconds, just until fragrant.
Char the noodles: add the remaining tablespoon of oil, then the noodles. Spread them out, leave them alone for 45 to 60 seconds so the bottom layer chars lightly, then toss. Pour the sauce around the edge of the wok so it sizzles on contact, and toss for 1 to 2 minutes until every ribbon looks glossy and evenly dark, with a few crisped edges. If the noodles feel stiff, sprinkle in a tablespoon of water and toss again.
Add the egg: push the noodles to one side, pour the beaten eggs into the empty space, and let them set for 20 seconds. Scramble them into soft ribbons, then fold them through the noodles.
Finish with sprouts and chives: return the shrimp and any juices to the wok. Add the bean sprouts and garlic chives and toss for 30 to 45 seconds, just until the sprouts lose their raw edge but stay crisp and the chives wilt. Taste and adjust, then serve right away on warm plates with lime wedges.