Add JSON Schema documentation for bambulab/v1, with real inheritance

Mirrors src/printer/bambu_commands/ as language-agnostic schema docs (not
used for codegen — the Rust structs stay hand-written). envelope.schema.json
is the base; get_version/pushall request+response schemas extend it via
allOf + $ref, which is genuinely closer to real inheritance than anything
else in this project's schema stack: proto and Rust only ever gave us
composition (embed a field holding the base struct), never a mechanism
that reuses a *referenced* schema's constraints automatically.

Verified with the jsonschema Python library, not just written by hand:
all three real examples from the conversation validate correctly, and —
importantly — negative tests confirm the base schema's required fields
and the command consts are actually enforced through the allOf chain, not
just documented. README covers the one real gotcha (additionalProperties:
false on a base schema silently breaks allOf composition, since allOf
validates each sub-schema against the whole instance independently rather
than merging them).

schemas/bambulab/v1/get_version.request.schema.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://continuum.internal/schemas/bambulab/v1/get_version.request.schema.json",
+  "title": "GetVersionRequest",
+  "allOf": [
+    { "$ref": "envelope.schema.json" },
+    {
+      "type": "object",
+      "properties": {
+        "command": { "const": "get_version" }
+      }
+    }
+  ]
+}