Compile root cfg from --override values without a config.yaml

[Volv-G]

A root @pipeline that loads config can now compile from --override values when the implicit default config.yaml is absent. This enables compile-time fan-out without a config file:

@pipeline("Sharded")
def sharded(cfg) -> None:
    for shard in cfg.shards:
        process.named(f"Process {shard}")(shard=shard)

tangle sdk pipelines compile sharded.py -o sharded.yaml --override 'shards=[us, eu]'

Behavior

Case	Result
Root, no config=, default config.yaml missing, ≥1 --override (cfg param and/or propagate_config=True)	Compiles from {} + typed overrides; warning names override keys only
Same, but no overrides	Error (unchanged), now hints at --override KEY=VALUE
Explicit config= missing (with or without overrides)	Error (unchanged)
Nested child missing its own config	Error (unchanged)
Overrides, no cfg and no propagate_config	Error (unchanged)

A missing default is modelled exactly as an empty config.yaml (raw_cfg={}, no config identity), so the compile key and output bytes match an empty file. For a propagate_config root the overrides become the broadcast payload; children still take only keys they declare.

Tests

New tests in tests/test_pipeline_compiler.py: list override → distinct named tasks (API + CLI), typed int → range(), missing-vs-empty key/bytes equivalence, strict cases above, propagate-only root broadcast. Full uv run pytest passes locally.