Add StreamSession passive subscribe tests and docs

[ajit-zer07]

Summary

• Pins macp-proto to 0.1.2 from crates.io so CI can build the passive-subscribe code landed earlier on this branch (previously a local path dep that CI cannot resolve).
• Adds unit and integration test coverage for the RFC-MACP-0006-A1 StreamSession passive subscribe flow introduced in src/{server,runtime,log_store}.rs.
• Updates docs/API.md, docs/sdk-guide.md, docs/testing.md, and README.md to describe the subscribe-only frame contract, authorization rules, and after_sequence
  semantics.

What changed

Dependency

• Cargo.toml / Cargo.lock — macp-proto = "0.1.2" (was a path dep to the spec repo).

Unit tests (src/server.rs, +8)

• subscribe_replays_session_history_from_zero
• subscribe_after_sequence_filters_history
• subscribe_unknown_session_returns_not_found
• subscribe_non_participant_is_forbidden
• subscribe_observer_identity_allowed
• subscribe_initiator_allowed_even_when_not_listed
• stream_request_with_envelope_and_subscribe_is_rejected
• subscribe_to_different_session_on_bound_stream_is_rejected

Integration tests (integration_tests/tests/tier1_protocol/test_passive_subscribe.rs, +6)

• History replay from sequence 0 and after a non-zero offset
• Unknown session terminates the stream with NotFound
• Non-participant caller receives an inline FORBIDDEN error frame; stream stays open
• Late-joiner replays history and then receives a live Evaluation
• Frame with both envelope and subscribe_session_id rejected with InvalidArgument
• New dev-dep: tokio-stream = "0.1" (for ReceiverStream on the request side)

Docs

• docs/API.md — extended StreamSession section with subscribe-only frame semantics and authz
• docs/sdk-guide.md — added passive subscribe to the StreamSession lifecycle steps
• docs/testing.md — references the new test_passive_subscribe.rs in the Tier 1 description
• README.md — new bullet under the StreamSession enabled block

Formatting

• cargo fmt --all in integration_tests touched two pre-existing files (tier1_jwt.rs, test_validation_gaps.rs). Whitespace only; these drifted because the main CI
  cargo fmt job does not traverse the integration_tests crate.

Test plan

• cargo fmt --all -- --check — clean in both crates
• cargo clippy --all-targets -- -D warnings — clean in both crates
• MACP_MEMORY_ONLY=1 cargo test --all-targets — all runtime unit + integration tests pass
• MACP_TEST_BINARY=../target/debug/macp-runtime cargo test --test tier1 -- --test-threads=1 — 67/67 pass (was 61)
• cargo build --release — succeeds
• cargo audit — exit 0
• CI passes on this PR (GitHub Actions ci.yml, integration-tests.yml)