TypeScript SDK: embed source and integrity sections in packed bundles

### Background #69295 adds `airflow-ts-pack`, which embeds the bundle manifest as a single first-line comment in `bundle.mjs` (`//# airflowMetadata=<base64 YAML>`); the Node coordinator reads only that head line (bounded `readline()`, 1 MiB cap). The Go SDK's AFBNDL01 executable bundle format provides two things the TS format does not (see the Trailer Layout section of `task-sdk/docs/executable-bundle-spec.rst`): - **Integrity validation** — a SHA-256 digest over the code region, verified by the scanner before the bundle is used. - **An embedded source region** — the original entrypoint (`.ts` / `.js`), so the UI can display the code; needed for the native TypeScript Dag feature (#69288). Review on #69295 agreed to keep that PR to the packing flow and refine the packing spec in a follow-up. Since `bundle.mjs` must remain directly runnable by `node`, the structure cannot be a binary EOF trailer like AFBNDL01; instead the head comment can carry the layout — a format version, start/end offsets for each section, and the digest. ### What needs to happen 1. Specify a versioned head-comment layout for `.mjs` bundles recording section offsets (bundled code, embedded source, metadata) and a SHA-256 digest of the code region, documented alongside AFBNDL01 in `task-sdk/docs/executable-bundle-spec.rst`. 2. Update `airflow-ts-pack` (`ts-sdk/src/cli/pack.ts`) to emit the new layout and embed the original entrypoint source. 3. Update the Node coordinator (`task-sdk/src/airflow/sdk/coordinators/node/coordinator.py`) to verify the digest during scan — log and skip on mismatch, with the same `(path, inode, mtime, size)` result caching as the AFBNDL01 reader algorithm — and expose the embedded source. ### Acceptance criteria - A packed `bundle.mjs` remains directly runnable with `node bundle.mjs`. - The coordinator logs and skips a truncated or modified bundle on digest mismatch. - The embedded entrypoint source is retrievable for display. - `node-bundle-spec.rst` documents the TS / J…