Name the usage a 51Did was created for, and take the raw byte surface off FodId

[jwrosewell]

Two changes to the same surface, in the order they were made.

1. Name the usage a 51Did was created for

FodId gains usage() (None, NonMarketing, Standard, Personalized) and usage_from_consent(), reading bits 0 to 2 and bit 3 of the flags byte. The usage bits are cumulative, so the accessor answers the highest usage granted rather than leaving callers to mask bits and misread every marketing identifier as non-marketing. Same accessor, same names, in all six 51Did packages. Found by the Trusted Server work, which must work only through this crate.

2. Take the raw byte surface off FodId

This is issue 39, raised on all six packages so the surface stays the same in every language. Once every bit has a name, the raw byte and the offsets are the remaining way to get it wrong, so they go.

• flags() is removed outright rather than made pub(crate). The typed accessors read the private field directly and the new unit tests are in the same module, so no accessor is needed anywhere.
• hash(), HASH_OFFSET and HASH_LENGTH are deleted, with the re-export block in lib.rs and the test that only proved the alias matched.
• The layout constants are pub(crate) and their lib.rs re-exports are gone, so pub use fodid::{FodId, IdType, Usage}; is now the whole typed surface. Two of the nine, PAYLOAD_LENGTH and RANDOM_PAYLOAD_LENGTH, are deleted instead, because nothing inside the crate reads them and pub(crate) would have left dead code under -D warnings.
• Rust never had a date minutes accessor and none is added. The other five packages are losing theirs, which brings them into line with this crate.

How the tests still build payloads

Rust integration tests are a separate crate and cannot see pub(crate), and a feature flag would have put the layout back in front of consumers. The tests therefore carry the layout themselves in fodid/tests/layout/mod.rs, taken from the specification rather than from the reader, which also removes a circularity, since a payload built from the reader's own constants agrees with the reader whatever either of them says. New unit tests in src/fodid.rs tie the reader's constants to the same published numbers. Where a test wanted the raw byte it reads fod_id.payload()[layout::FLAGS_OFFSET].

fodid-cloud

fodid-cloud/tests/cloud_fodid.rs asserted flags() and now asserts the usage, the consent bit and the type. Two doc comments there listed "flags" among the fields a caller reads and are reworded. No code in fodid-cloud/src changed, and nothing in cloud-request-engine was touched.

The specification

The byte structure is now specified once for all six languages, on specifications 25, at Identifier layout with the surface each package exposes at Package surface. The layout table was repeated in the crate documentation, the FodId doc comment and the README, and all three now name those pages as the authority. AGENTS.md still pointed at FodId::hash from before the match key rename and is corrected. Those links resolve once that pull request merges.

One thing added that is not the issue

fodid/Cargo.toml names reqwest-fetch in its checked cfg list. The OWID source is copied into this crate as a private module, feature gates and all, and SWAN-community/owid-rust 10 puts the ready made transport behind a feature of that name, so the copied mod.rs gains three gates naming it. Building this crate against that branch turns them into three errors under cargo clippy -- -D warnings, which is what continuous integration runs here, even though nothing in this crate enables the feature or reaches the network.

Checked by pointing the submodule at that branch, running ci/copy-owid-source.ps1 and building: three errors before the line, clean after it, with cargo build -p fiftyone-fodid-cloud --no-default-features --target wasm32-wasip1 still succeeding. Naming the value costs nothing while no copied file carries it, and it means the submodule pointer can move later without taking the build down.

Verification

cargo build --all-targets     exit 0
cargo test                    exit 0   67 suites, 542 passed, 0 failed, 7 ignored
cargo fmt --check             exit 0
cargo clippy --all-targets -- -D warnings   exit 0
cargo doc --no-deps -p fodid -p fiftyone-fodid-cloud   clean

The seven ignored are the pre-existing live cloud and network tests. The crate needs the OWID copy, so ci/copy-owid-source.ps1 was run first. The documentation build is included because removing public items breaks intra-doc links.

Closes #39

Produced with AI assistance and needs human review.