Wire up proto/bambulab/ build pipeline (boilerplate — message bodies TBD)

Adds bambulab/v1 and bambulab/v2 to the prost-build pipeline, scoped
separately from continuum.v1's camelCase convention: bambulab.* uses
snake_case, matching Bambu's actual MQTT wire JSON exactly, since this is
edge-only traffic (continuum-proxy) that has to match the printer's real
format rather than a schema we get to design.

Verified the scoping doesn't collide: type_attribute calls are cumulative,
so the existing '.' blanket selector had to become '.continuum' specifically
- keeping it global would stack a second, conflicting #[serde(rename_all)]
onto every bambulab type too and fail to compile.

No message bodies added - proto/bambulab/{common,v1/*,v2/*}.proto are
intentionally just syntax+package+guidance comments (matching what was
already started for v1). build.rs has inline notes on the two real gotchas
already hit while prototyping this: the field_attribute path-matching
oneof/variant collision (solution: hand-write the envelope enum in Rust
instead of modeling it as a proto oneof), and serde needing #[serde(default)]
on fields that aren't always present.

proto/bambulab/v2/request.proto

@@ -0,0 +1,8 @@
+syntax = "proto3";
+
+package bambulab.v2;
+
+// import "bambulab/common.proto"; once common.proto has a shared message
+// worth embedding (composition, not inheritance — see that file).
+
+// V2-generation (X1/H2 series) command messages go here.