Declare proprietary licensing and enforce permissive-only dependencies

Continuum is intended to be sold, so every dependency has to permit
distribution in closed-source commercial software. Nothing recorded that
before: our own crates and npm packages carried no licence field at all,
which made `cargo deny` report them as `unlicensed` errors and left the
actual policy nowhere.

Crates now declare `LicenseRef-Proprietary` with `publish = false`.
That string is deliberately not a real SPDX identifier, so it must never
appear in a `deny.toml` allow list - a third-party crate claiming it
would be a genuine finding. npm packages use `"license": "UNLICENSED"`,
the equivalent convention there.

`deny.toml` holds the permissive-only allow list. The policy itself, the
audit, and the three categories cargo-deny cannot see (external
binaries, ML model weights, self-hosted servers) are documented in
continuum-common's docs/LICENSES.md.

deny.toml

@@ -0,0 +1,36 @@
+# Licence policy for a closed-source commercial product.
+# See docs/LICENSES.md in continuum-common for the rationale and the
+# non-cargo dependencies (external binaries, ML models) this file cannot see.
+#
+# Run: cargo deny check licenses
+
+[licenses]
+# Permissive only. Anything copyleft reaching a shipped binary would make
+# the product undistributable without releasing our source.
+allow = [
+    "MIT",
+    "Apache-2.0",
+    "Apache-2.0 WITH LLVM-exception",
+    "BSD-2-Clause",
+    "BSD-3-Clause",
+    "ISC",
+    "Zlib",
+    "Unicode-3.0",
+    "Unlicense",
+    "MPL-2.0",
+    "CC0-1.0",
+    "BlueOak-1.0.0",
+]
+confidence-threshold = 0.9
+
+# Our own crates declare `LicenseRef-Proprietary`, which is deliberately not
+# an SPDX identifier and must never appear in the allow list above - a
+# third-party crate claiming it would be a real finding. They are recognised
+# as first-party by `publish = false` in their manifests.
+private = { ignore = true }
+
+[bans]
+deny = []
+
+[advisories]
+yanked = "deny"