feat: add zero_amount_carve_out helper

[vvillait88]

Summary

• Skip upstream verify+settle for $0 orders (redemption-code flow). CDP rejects EIP-3009 `value=0` as `invalid_payload`; pympp's tempo intents reject the `proof` payload for $0 settles. The helper parses the credential and lifts the signer for wallet-capture attribution.
• Returns `ZeroSettleResult{ signer_address, signer_network, tx_hash=None }`. Same shape as `process_x402_settle` success so callers can branch on rail, not on shape.
• For `rail="x402-base"`: pass `payload` (verified x402 dict). For `rail="tempo"`/`"solana"`: pass `authorization_header` (Payment <base64>).
• Exported from `agentscore_commerce.payment`.

Tests

• 14-fixture corpus locked as the cross-language contract with `@agent-score/commerce`'s `zeroAmountCarveOut`.
• Parametrized via `pytest.mark.parametrize` so multiple drifts surface independently.
• Invariants: `tx_hash` always `None`; rail dispatch ignores irrelevant args.

Test plan

• `uv run pytest tests/test_zero_settle.py` — 18 tests pass
• `uv run pytest` — full suite 981 pass / 3 skip, coverage 95.16%
• `uv run ruff check`, `uv run ruff format --check`, `uv run ty check` — green

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