feat(errlanes): enum-level shared rejection code

[bodymindarts]

Implements §1 of the handoff handoff-errlanes-shared-rejection-code-and-unique-key-ensure (es-entity-dev space).

• #[rejection(code, description, level)] on an enum is the default for every variant without its own code / code_and_level_from / delegate.
• Code type: one variant per distinct code (named after the enum); CODES lists a shared code once.
• Lift arms under a shared-code enum take the enum's code.
• Compile errors: unused shared code, empty enum, with code_prefix, variant description under a shared code.
• #[rejection(from)] on several variants was already allowed (one From per variant); now tested.
• README section "One code, several causes" with both lana examples as doc-tests (local stand-ins).

Decisions (handoff §4), all recommended options taken:

• D1: recommended option taken (empty enum with code is a compile error)
• D5: recommended option taken (lockstep 0.20.0 with PR 2; no version bump here)

Deviations: the handoff asks for a book mirror in an "errlanes chapter", but the book has no errlanes chapter, so only the README (the crate docs) is updated. An explicit code_and_level_from counts as declaring its own code. No CHANGELOG edit (git-cliff generates it from this title).

Pre-existing: cargo clippy --all-targets -D warnings reports lints in untouched errlanes tests (codes.rs, lift_into.rs, lift_newtype.rs, lift_struct.rs).

🤖 Generated with Claude Code

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Note

Medium Risk
Changes proc-macro code generation for rejection metadata and lift/catalogue behavior; mistakes could mis-report API error codes, though coverage is strong and misuse is mostly caught at compile time.

Overview
Adds enum-level #[rejection(code, description, level)] on Rejection derives so every variant without its own code, delegate, or code_and_level_from reports the same rejection code (and shared description/level, with per-variant level still allowed).

The derive emits a single code-type variant named after the enum for that shared string, deduplicates it in CODES, and does not auto-forward lift metadata from source variants when shared code applies—lifted arms use the family code instead. Variants that keep their own code or delegate unchanged can mix with shared-code variants.

Compile-time guards reject unused enum-level code, code on empty enums, code together with code_prefix, per-variant description under shared code, and a self-coded variant whose name matches the enum (shared code owns the enum-named code variant).

Docs add a README “One code, several causes” section; trybuild compile-fail cases and integration tests cover catalogue shape, display/source, lift, compose, and mixed delegate/shared families.

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