fix(spawn): bind every real positional arg of a spawned callable (calque#191)

[scttfrdmn]

Summary

• SpawnCallable.MethodArg only ever carried the FIRST non-self/cls parameter name — a callable like AI-Almanac's forecasts_app.py's run_forecast_inference(job_id, model_id, config) would (once .spawn() call site via a dynamic subscript (dict[key].spawn(...)) resolves to an empty target — spawnrun can't drive it #189/fix(parse): recognize .spawn() on a dict-of-functions Subscript (calque#189) #190/fix(parse): classify every spawn candidate as InvokeSpawn (calque#189 follow-up) #192's target-resolution fix landed) build a real shard, but silently leave model_id/config undefined inside the worker on real hardware. Same class of arity gap calque#187 fixed for run/real/fleetrun; spawn-run's own separate driver had no equivalent.
• New SpawnCallable.MethodArgs carries the full non-self/cls arg list (ResolveSpawnCallables/ResolveOneSpawnCallable populate it via a new nonSelfCls helper).
• spawnManifestBody (new, factored out of runSpawnShard for testability) routes a 2+-arg callable through the SAME Starmap/MethodArgs splat mechanism .starmap() already uses in runner.py — spawnArgsPayload was already building the right list payload; nothing downstream consumed it as a splat until now. A single-arg callable (the common case) is byte-for-byte unchanged.

Verification

Went beyond manifest-field inspection: TestSpawnDictDispatchMultiArgBindingEndToEnd drives the real fixture's bundle(job_id, config) through the actual warm.Supervisor + runner.py subprocess and asserts the actual returned values for both args — mirroring TestStarmapEndToEndDryRun's own proof strength (real execution, real result, not just "didn't crash" or "field is set"). Also extended spawn_dict_dispatch.py's fixture to return both args so this assertion is possible.

Files touched

• internal/exec/spawnshard.go — SpawnCallable.MethodArgs, nonSelfCls, updated ResolveSpawnCallables/ResolveOneSpawnCallable.
• internal/exec/spawnshard_test.go — new unit tests for MethodArgs population (plain function + class method, self-stripping).
• cmd/calque/spawnrun.go — new spawnManifestBody pure function, wired into runSpawnShard.
• cmd/calque/spawnrun_test.go (new) — unit tests for spawnManifestBody's splat-routing decision.
• cmd/calque/spawn_dict_dispatch_e2e_test.go — new real-execution end-to-end test.
• testdata/scripts/spawn_dict_dispatch.py — fixture extended to return both args.
• CHANGELOG.md.

Test plan

• go build ./... && go vet ./... && gofmt -l . && go test -count=1 ./... — all pass
• golangci-lint run ./... — 0 issues
• ruff check . — all checks passed
• TestSpawnDictDispatchMultiArgBindingEndToEnd — real subprocess execution, confirms both job_id and config bind to their real supplied values