conformance: add check-decode --format pb-text so the cross-port text gate runs Python #37

Problem

The cross-port text gate (scripts/cross_text_check.sh in protowire) lists python in NOT_YET. Python has no check-decode --format pb-text, so nobody checks the PXF that pxf.marshal writes against protowire-go's goldens in testdata/text/cases/. This issue is #35's twin: #35 covers the write gate, this one the text gate.

Python's marshaller is protowire-cpp's. cpp is held to the gate already, including the Annotated cases since cpp#129 (protowire c48fcc0). So once the pin is on v2.0.0 (#31), a pb-text mode should pass with little or no bridge work. That is inferred, not measured.

Harness constraint (shared with #35)

check_decode.py reads a .binpb descriptor set next to --proto, because Python cannot compile a .proto. Only testdata/adversarial ships one. The text and write gates pass --proto testdata/text/text.proto, which has no .binpb. The harness needs to build one, for example with protoc --include_imports when the file is missing. Or the spec repo ships one. (In this session's probe, protoc -I . -I proto --include_imports built working sets for nulls/ and keyed/.)

Done when

• check_decode.py --format pb-text reads PB with the stock runtime and writes pxf.marshal's text on stdout.
• cross_text_check.sh runs Python with no new exclusions, or each exclusion has an issue.
• A protowire PR moves python out of NOT_YET and into the text workflow's REQUIRE_PORTS.