## Why
`ContextualUserFragment` needs to be usable behind `dyn` for render-only
paths, but associated constants made the trait non-object-safe.
## What changed
- Replaced associated constants with trait methods so `dyn
ContextualUserFragment` can render fragments.
- Preserved the existing typed `T::matches_text(text)` registration
pattern via `type_markers()`.
- Kept default `render()` on the main trait so implementations only
provide role, markers, and body.
- Added unit coverage for rendering a `Box<dyn ContextualUserFragment>`.
## Verification
- `cargo test -p codex-core contextual_user_fragment_is_dyn_compatible`
- `just fix -p codex-core`
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