Model filesystem read outcomes without fusing transport

[briansrls]

Summary

Model Filesystem.Read success and refusal as the FilesystemReadOutcome coproduct through a pure .dag fold over the existing scalar transport channels. Convert gunbc.ci_yaml_validate as the first production consumer, preserving read refusal separately from YAML parse refusal before its existing generated-artifact policy projects either refusal to false.

The adoption standing is deliberately mitigatable, not mechanically preventable. Its subject is unconverted consumption: a bound read result projected outside the arguments of filesystem_read_outcome. Baseline measurement at origin/main b21b710d5378387ae0c841f07323292c5d72faba found 92 unconverted consumption sites across 48 files. This PR routes one through the fold; 91 unconverted sites across 47 files remain. Calls and their three projections remain because they supply the fold, so they are not the adoption denominator. The standing explicitly excludes its own prose carrier from enumeration and defines the reachable next rung as zero unconverted consumption sites.

The compiler-only filesystem_read intrinsic is a separate replacement migration and is untouched. No service grammar or Rust realization change is needed: scalar channels remain the wire observation, while the interface authority owns their modeled fold. The branch has merged current main; the PR's three-dot diff is exactly three .dag files and contains no v1 compiler changes.

Test plan

• ctrl-build --remote -- bash -lc 'cargo run --release -p v1-compiler --bin gunbc -- run --source-root dag --source-root src/v2 --entry dag/test/claim/filesystem_read_outcome_witness_test.dag --function filesystem_read_success_carries_content --claim-run && cargo run --release -p v1-compiler --bin gunbc -- run --source-root dag --source-root src/v2 --entry dag/test/claim/filesystem_read_outcome_witness_test.dag --function filesystem_read_refusal_carries_error --claim-run && cargo run --release -p v1-compiler --bin gunbc -- run --source-root dag --source-root src/v2 --entry dag/test/claim/filesystem_read_outcome_witness_test.dag --function ci_yaml_read_refusal_is_not_parse_refusal --claim-run' — all three PASS after merging current main.
• git diff --check — PASS.
• Pre-push cargo fmt --all --check — PASS.

[gunbai-bot]

Duplicate auto-opened from the source branch after #9265 was squash-merged. The implementation is already delivered by #9265; this draft must not be reviewed or merged. — sent from zesty-newt-651