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/envelope.schema.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://continuum.internal/schemas/bambulab/v1/envelope.schema.json",
+  "title": "CommandEnvelope",
+  "description": "Fields every Bambu MQTT command/response shares. This is the 'base class' — other schemas extend it via allOf + $ref. JSON Schema doesn't have inheritance either, strictly speaking (no 'extends' keyword) — allOf is composition: it says the instance must satisfy THIS schema AND whatever else is listed alongside it. It's the closest thing to real inheritance ergonomics of anything we've used across this whole schema stack (proto, Rust), because $ref + allOf lets you name and reuse a shape without re-declaring its fields, which proto/Rust composition can't quite do (there you still write `common: PrinterReportCommon` by hand on every struct).",
+  "type": "object",
+  "properties": {
+    "sequence_id": { "type": "string" },
+    "command": { "type": "string" }
+  },
+  "required": ["sequence_id", "command"]
+}