Document raw-mode support per engine

[karust]

Context

Contributors and users should not have to inspect server wiring to know which engines support browser mode, raw mode, and parse endpoints.

Scope

Add a small docs table for current engine support across browser mode, raw mode, and /{engine}/parse.

Done when

• The table matches current code.
• README links to the table from the search endpoint section.
• make test passes.

Contributor notes

Start with cmd/serve.go, README.md, and docs/CONTRIBUTING.md.