Add Autodesk cloud sync connector

Initial implementation of the ifcviewer-autodesk connector — a separate process that bridges the IfcViewer to Autodesk APS (BIM 360 / ACC). Speaks JSON-RPC 2.0 over stdio per CLOUD_SYNC_PROTOCOL.md (also added). PKCE OAuth with keyring-backed token storage, customtkinter browse/picker UI, and PyInstaller packaging.

Implements both interactive and non-interactive variants of each push/pull (pull_ifcfed[_interactive], pull_models[_interactive], push_ifcfed[_interactive], push_model[_interactive]) so the viewer can offer both "Save"/"Open from Cloud" and "Save As"/"Add Model from Cloud" entry points. File transfers report progress through a dialog with per-byte updates; pull_models shows "(i/N)" for batches.

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src/ifcviewer-autodesk/README.md

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+# `ifcviewer-autodesk`
+
+Autodesk Forma (APS / Docs) connector for IfcViewer.
+
+Implements the JSON-RPC connector contract defined in
+[`CLOUD_SYNC_PROTOCOL.md`](CLOUD_SYNC_PROTOCOL.md). The connector is a separate
+process the viewer launches and speaks to over stdio.
+
+UI is built on **CustomTkinter** (Tcl/Tk under the hood), keeping the
+packaged connector around 50 MB unpacked / 21 MB zipped on Linux. The Python
+running this code must include `tkinter` (most distribution Python builds do;
+on Gentoo make sure `USE="tk"` is set for `dev-lang/python`).
+
+## Install
+
+```bash
+cd src/ifcviewer-autodesk
+python -m venv venv
+source venv/bin/activate
+pip install -e .
+```
+
+## Run
+
+```bash
+ifcviewer-autodesk
+```
+
+The connector launches without any configuration; on first run, invoke
+`open_settings` (or, equivalently, set the `APS_CLIENT_ID` env var) to
+configure the Autodesk client id.
+
+Then send newline-delimited JSON-RPC 2.0 requests on `stdin`. Examples:
+
+```json
+{"jsonrpc":"2.0","id":"0","method":"open_settings"}
+{"jsonrpc":"2.0","id":"1","method":"pull_ifcfed_interactive"}
+{"jsonrpc":"2.0","id":"2","method":"pull_models","params":[{"display_name":"foo.ifc","id":"abc","source":{"connector":"autodesk","hub_id":"b.hub","project_id":"b.proj","item_id":"urn:adsk...","version_id":"latest"}}]}
+{"jsonrpc":"2.0","id":"3","method":"push_ifcfed_interactive","params":{"path":"/tmp/project.ifcfed"}}
+{"jsonrpc":"2.0","id":"4","method":"push_ifcfed","params":{"path":"/tmp/project.ifcfed","manifest":{"connector":"autodesk","hub_id":"b.hub","project_id":"b.proj","item_id":"urn:adsk..."}}}
+```
+
+The viewer is expected to launch this binary once per session and keep it alive
+until shutdown; closing the connector's `stdin` triggers a clean exit.
+
+### Configuration
+
+The connector reads the Autodesk client id from two places, in order:
+
+1. The `APS_CLIENT_ID` environment variable (takes precedence — useful for dev
+   overrides).
+2. `<config dir>/settings.json` (persisted via the settings dialog).
+
+The config directory is platform-specific:
+
+- Linux: `~/.config/ifcviewer-autodesk/`
+- macOS: `~/Library/Application Support/ifcviewer-autodesk/`
+- Windows: `%APPDATA%\ifcviewer-autodesk\`
+
+OAuth tokens are stored in the OS keychain (Secret Service on Linux, Keychain
+on macOS, Credential Manager on Windows), keyed by the client id, so changing
+the client id starts a fresh session.
+
+## Cache
+
+The connector owns its own cache. Resolved files live under (Linux):
+
+```
+~/.cache/ifcviewer-autodesk/
+  ifcfeds/<hash>/<name>.ifcfed[.manifest]
+  models/<hash>/<filename>
+```
+
+Each file is the sole child in its directory so the viewer can write sidecar
+files (e.g. `.ifcview`) next to it without colliding. A new resolved version of
+a model lands in a fresh `models/<hash>/` directory; the old directory may be
+removed manually to clear space.
+
+## Status
+
+What is implemented:
+
+- Strict JSON-RPC 2.0 host over stdio
+- APS PKCE sign-in with keyring-backed token store
+- Hub / project / folder browsing (Qt UI)
+- `pull_ifcfed_interactive`, `pull_ifcfed`, `pull_models`, `pull_models_interactive`
+- `push_ifcfed_interactive`, `push_ifcfed`, `push_model_interactive`, `push_model`
+- `open_settings` — edit the client id, sign out
+- Connector-managed cache with sole-child invariant
+- Adjacent `.ifcfed.manifest` written/read alongside `.ifcfed` files
+
+What is intentionally not implemented:
+
+- JSON-RPC notifications for progress streaming (the connector shows its own
+  progress dialog instead, per spec)
+- Cancellation of in-flight downloads
+- Subdirectory upload layouts inside push destinations (one flat file at a time)