Fix duplicate Studio asset URI scheme

[revvy02]

Summary

The Display impl for AssetRef already prepends rbxasset:// for Studio variants and rbxassetid:// for Cloud variants:

impl fmt::Display for AssetRef {
    AssetRef::Cloud(id)    => write!(f, "rbxassetid://{id}"),
    AssetRef::Studio(name) => write!(f, "rbxasset://{name}"),
}

The studio backend was packing those prefixes inside the inner string as well, so codegen ended up emitting doubled URLs:

-- output/assets.luau on `asphalt sync studio` against current main:
local assets = {
    ["test1.png"] = "rbxasset://rbxasset://.asphalt-myproject/<hash>.png",
}

For models/animations with a lockfile entry (i.e. already cloud-uploaded), the result was even more confused:

["my_model.rbxm"] = "rbxasset://rbxassetid://12345"

Fix

• For locally synced files, drop the rbxasset:// prefix from the inner string of AssetRef::Studio(...) so Display adds it once.
• For cloud-uploaded models/animations, return AssetRef::Cloud(entry.asset_id) so the correct rbxassetid://{id} is produced via Display.

Test infrastructure

This PR also adds an ASPHALT_TEST env var that routes the studio sync folder to params.project_dir instead of the Roblox Studio content path. This is test-only plumbing — the existing common test harness (tests/common/mod.rs) already sets ASPHALT_TEST=true in Project::run(), but the studio backend ignored it. With the env var honored, integration tests for sync studio no longer require a Roblox Studio install.

Regression test

Added studio_sync_emits_single_rbxasset_prefix in tests/sync.rs. It runs asphalt sync studio and asserts that the generated output/assets.luau:

• Contains rbxasset://.asphalt-test/ (single prefix, correct format)
• Does NOT contain rbxasset://rbxasset://
• Does NOT contain rbxasset://rbxassetid://

I verified the test fails on current main (captured output: ["test1.png"] = "rbxasset://rbxasset://.asphalt-...") and passes with the fix.

Test plan

• cargo build clean on rc.5
• cargo test — 6/6 unit + 11/11 integration pass
• New regression test fails on main without the fix

[jackTabsCode]

Hey. Thanks for the fix, but I don't think we need these tests. Do you mind reducing your PR to the few lines of code necessary to fix us adding that prefix?

[revvy02]

removed tests, just the minimal fix now