Add batch mode for processing multiple inputs via JSONL

[dnagoda]

📚 Stack (bottom → top)

1. Add batch mode for processing multiple inputs via JSONL #1 Add batch mode for processing multiple inputs via JSONL
2. Preserve JSON input object order #2 Preserve JSON input object order
3. Refine batch failure handling #3 Refine batch failure handling
4. Reuse the compiled provider across runs #4 Reuse the compiled provider across runs
5. Compute humanized bytes lazily #5 Compute humanized bytes lazily
6. Add minimal batch output with --batch-full-output opt-in #6 Add minimal batch output with --batch-full-output opt-in

Stacked draft PRs on the fork; review bottom-up. Merge target is Shopify/function-runner once upstream write access is available.

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Why

The function-runner is used by function-rerunner to replay production
function inputs for Checkout Blocks migration parity. Replaying hundreds of
thousands of inputs one process-per-input is dominated by process startup and
Wasm engine/module compilation, which is identical work repeated for every row.

What

Add a --batch mode that reads newline-delimited JSON (JSONL) inputs from a
file (--input) or stdin and runs the function once per line:

• Creates the Wasmtime Engine and loads the Module once, then reuses
  them across every input row — the expensive setup is amortized.
• Infers the codec (JSON vs. MessagePack) once from the module's imports.
• Streams one compact JSON result per line to stdout (JSONL out), so callers
  can parse results incrementally.
• --batch-continue-on-error keeps processing on per-line read/parse/execute
  failures, emitting a {"success":false,"error":...} record for the failing
  line; without it, batch mode fails fast on the first error.
• Prints a Batch complete: N processed, S successful, E errors summary to
  stderr so it never contaminates the JSONL on stdout.

Single-run behavior is unchanged — the existing path is factored into
run_single_mode and shares the same engine/module setup.

Testing

• cargo test
• cargo build --release

[dnagoda]

Moved upstream to Shopify#589 now that write access is available. Closing this fork PR.