Rename the four Reader types so only one reads bytes

[mfw78]

What does this PR do?

Renames the three point-lookup handles that shared the Reader name with the one type that reads bytes, so Reader means one thing.

Changes

• nectar-mantaray: Reader<L> becomes TrieLookup<L> in the Trie* family, ReaderError becomes LookupError, and the public reader module moves to lookup.
• nectar-ldb: Reader<S, F, R> becomes KeyLookup<S, F, R>, ReaderError becomes LookupError, the private reader module and the reader integration-test file move to lookup, and View's private reader() helper becomes lookup().
• nectar-feeds: Reader<S, BODY_SIZE> becomes FeedView<S, BODY_SIZE>, the private reader module moves to view, and the Feed geometry comment now names the view and the publisher.
• The nectar-file byte reader keeps the name; its re-exports and every consumer of it are untouched.
• The manifest-test imports drop their as MantarayReader and as LdbReader aliases, which existed to disambiguate two types that were both called Reader and now name themselves.
• Module and type docs state lookup where they said reader; local bindings named reader keep their role name, the way the existing cursor locals do against ScanCursor and TrieCursor.

Breaking changes

Pre-1.0 API, so these land as-is: three public types are renamed (mantaray Reader, ldb Reader, feeds Reader), two error types are renamed (the ldb and mantaray ReaderError), the public path nectar_mantaray::reader becomes nectar_mantaray::lookup, and the LdbFormatError and TrieFormatError Read variants now wrap LookupError.

Testing

• Unit tests pass: cargo nextest run --workspace (1429/1429) plus every per-feature nextest lane, every doctest lane, and the wasm32 checks from unit.yml, on a warm target.
• cargo clippy clean on the full 17-lane feature matrix from lint.yml, with machete, the fmt checks, the no-em-dash, serde-dev-only, reinvention and arbitrary-proptest-dev gates, and rustdoc under its docs.rs feature set for the three touched packages.
• Manual testing completed: none; the change is a mechanical rename witnessed by the suites above.
• Documentation updated: the module docs of all three crates state the settled names.

Related issues

This PR is the Reader half of the rename; the issue's Cursor half already landed with the cursor work.

AI assistance disclosure

AI Assistance: opencode used for the collision check, the mechanical rename and this PR body; the names were settled in discussion with the maintainer.

Notes for reviewers

• The names were checked against the tree before use, as the issue warns: the cursor-flavoured alternatives are dead, because the shared read machinery already landed PathCursor and RawCursor, mantaray's manifest module already owns TrieCursor, and none of the three handles walks sequentially, so a cursor name would re-create the confusion the issue removes.
• TrieLookup follows the backend-qualifier convention the cursor work settled for mantaray (TrieListing, TrieCursor, TrieView), and the crate's own docs call the handle a depth-guarded point lookup.
• KeyLookup is the issue's own description of the ldb handle, and it sits beside the View that speaks the map vocabulary over it; FeedView is the issue's description of the feeds handle, the read mirror of Publisher.

Checklist

• Code follows project style
• Self-review completed
• Tests added/updated
• No console.logs or debug code left behind
• PR title is descriptive

Closes

• Closes workspace: rename the four Reader types so only one reads bytes #837