test: add sessions-canary fixture for host integration of act:sessions

Hand-rolled minimal component exporting both \`act:tools/tool-provider\`
and \`act:sessions/session-provider\`. Each session holds a \`u64\`
counter; tools \`read\` and \`increment\` operate on the counter
identified by \`std:session-id\` in call metadata.

This is a stand-in until act-sdk-rs grows session macros — at which
point we'd write a proper SDK-driven component. The canary lets us
implement and test session-provider host bindings against a real
.wasm artifact without depending on SDK work.

Layout:
- act-cli/tests/fixtures-src/sessions-canary/ — source crate (own
  workspace, not part of act-cli's)
- act-cli/tests/fixtures/sessions-canary.wasm — packed binary tracked
  in git for tests

Verified the existing host (basic ActWorld with tool-provider only)
loads the canary fine — extra exports (session-provider) are not a
mismatch error in wasmtime. \`act info --tools\` lists the tools, and
calling tools without std:session-id returns \`std:invalid-args\` as
expected; calling with an unknown session-id returns
\`std:session-not-found\`.

Author: GamePad64

act-cli/tests/fixtures-src/README.md

@@ -0,0 +1,27 @@
+# Test fixture sources
+
+Hand-rolled component sources whose built wasms live in `tests/fixtures/`.
+
+These are NOT production components — they are minimal artifacts written
+to exercise host integration code paths (especially capability interfaces
+that don't yet have SDK ergonomic support).
+
+## sessions-canary
+
+Stateful counter component. Exports both `act:tools/tool-provider` and
+`act:sessions/session-provider`. Each session holds a `u64` counter;
+tools `read` and `increment` operate on the counter identified by
+`std:session-id` in call metadata.
+
+### Rebuild
+
+```bash
+cd tests/fixtures-src/sessions-canary
+cargo build --target wasm32-wasip2 --release
+# Pack metadata into the wasm:
+cargo run --manifest-path ../../../../act-build/Cargo.toml --release -- \
+    pack target/wasm32-wasip2/release/sessions_canary.wasm
+# Copy into the fixtures dir:
+cp target/wasm32-wasip2/release/sessions_canary.wasm \
+    ../../fixtures/sessions-canary.wasm
+```