Pin rbx-dom crates to released versions

[Lauel09]

The rbx-dom dependencies track branch = "master":

rbx_binary  = { git = "https://github.com/rojo-rbx/rbx-dom/", branch = "master" }
rbx_dom_weak = { git = "https://github.com/rojo-rbx/rbx-dom/", branch = "master" }
rbx_reflection = "3"

Any breaking change upstream lands here immediately with no version to fall back to, and that has now happened — the crate doesn't compile against current master. This switches to published releases and migrates the API.

Changes

• rbx_dom_weak 4, rbx_reflection 7, rbx_binary/rbx_xml 3, all from crates.io
• Properties are keyed by Ustr, so Source is looked up with ustr("Source")
• Instance::class is a Ustr; to_string() where a String is needed
• rbx_reflection::get_class_descriptor no longer exists — classes are looked up in a ReflectionDatabase, and services are detected with tags.contains(&ClassTag::Service)
• Added rbx_reflection_database, preferring the locally installed database (kept current by tools like Rojo) and falling back to the bundled one, so newer classes resolve on machines that have it
• rbx_binary::from_reader_default → from_reader

No behaviour change intended; this is a dependency and API migration.

Testing

cargo check is clean for both parcel-lib and the src-tauri crate. Converted a 3 MB .rbxl (~200 scripts, ~80k instances) and diffed the extracted sources against the ones read out of the place file directly — they match.

src-tauri/gen/schemas/* also regenerate on build; I left those out of this PR since they're build artifacts.