fix(control): native gateway session txn_id for in-txn SQL reads

[pizofreude]

What & why

Two small Control Plane fixes so native-protocol transactions stage and read correctly when dispatch goes through the gateway.

1. fix(control): ensure native session on first-frame OpCode::Begin

   • OpCode::Begin as the first post-handshake frame could no-op: SessionStore::begin only mutates an existing session entry, and SQL-path ensure_session had not run yet.
   • Call sessions.ensure_session at the top of handle_begin (SQL BEGIN already ensures the session in sql.rs before calling here).

2. fix(control): thread session txn_id through gateway local dispatch

   • Native SQL / direct-op reads inside an explicit transaction build a gateway QueryContext and call Gateway::execute.
   • The gateway previously hard-coded txn_id: None into DispatchRouteParams, and local dispatch used the txn-less SPSC path, so the Data Plane never resolved the per-transaction staging overlay.
   • Add QueryContext.txn_id, pass it into dispatch, and use dispatch_to_data_plane_with_txn for local routes. Native seams that hold a session txn stamp it; autocommit / system callers keep None.

This is the tip-based follow-up after closing #176 (obsolete wire groundwork). No WIRE_VERSION bump — ExecuteRequest.txn_id already exists on main. No internal roadmap labels in source or tests.

How to test

cargo test -p nodedb --test native_gateway_txn_overlay
cargo test -p nodedb --test native_transactions_staging --test native_direct_op_txn_overlay
cargo fmt --all --check
cargo clippy -p nodedb --all-targets --all-features -- -D warnings

Locally verified: both new tests green; staging + direct-op overlay suites green; cargo check -p nodedb --lib and clippy -D warnings clean.

Tradeoffs / alternatives considered

• Did not reopen feat(control): native SQL in-txn read-your-own-writes (U3) + cross-cluster txn_id groundwork #176 — that branch was 160+ commits behind and mixed a wire bump for an inert field with the useful single-node fixes.
• Out of scope: materializing staged overlays on remote nodes for cross-node in-txn reads. This PR makes the gateway carry txn_id locally (and on the remote RPC field when present); full cross-node overlay reachability is separate work.
• Streaming path receives task.txn_id on the context for consistency; execute_stream still does not fully consume session overlays end-to-end (same as prior streaming gap).

Notes for review

• Control Plane only; plane separation unchanged.
• Behavior-named tests only (native_first_frame_begin_buffers_following_write, native_in_txn_sql_select_reads_own_write).
• Please add the run-ci label if CI is still opt-in.