feat: add CDS (SAP CAP) language support (.cds)

[nickels]

Part of #648 (the language support tracking issue, which consolidates individual requests). No separate CDS request issue exists.

Indexes SAP CAP CDS models (.cds) the way they are written: namespaces and contexts, services, entities and views, elements with their associations and compositions, types and enums, aspects, events, actions and functions (bound and unbound), plus using imports with alias expansion, extend and annotate directives, and annotations kept as decorators (@readonly, @requires, @restrict, @path, @UI.*, ...). Service projections link to the db entities they project, includes link to the aspects they pull in, associations link to their targets, and relative using ... from paths link files.

This is a from-scratch extractor on current main, modelled on #1685 (Elixir): definitions are dispatched in visitNode because CDS definitions carry their elements as direct children, and a small CDS branch in the name matcher resolves references by CDS scoping rules (service scope, then enclosing context and namespace, then fully qualified) using the dot-normalized qualified name.

What it extracts

• namespace / context (namespace), service (module, members marked exported)
• entity / view (class) including as projection on and as select from with join sources
• elements (field) with signatures, key / virtual decorators, nested structures, enum members
• type (type_alias or enum), aspect (interface), event (struct), annotation definitions
• unbound action / function (function), bound actions { } (method)
• using { a.b.C as D } from '../db/schema' alias maps, per-artifact references and file import edges
• extend X with { ... } elements and actions rooted at the extended definition, annotate X with references
• doc comments (/** */, //) as docstrings

Grammar

Vendored cap-js-community/tree-sitter-cds @ 66bc383 (v2.0.0, Apache-2.0) with a small local patch (docs/grammars/tree-sitter-cds.patch): empty [] / {} annotation values and annotations after default / not null, which broke 9 of 247 .cds files across 11 real CAP repos. Built with tree-sitter-cli 0.25.10 (generate + build --wasm --docker, emsdk 4.0.4, ABI 15); all 60 upstream corpus tests plus 2 new ones pass on the regenerated parser, and the patched wasm parses all 247 corpus files with no ERROR node. SAP archived the grammar repository in June 2026 and it describes itself as a highlighting grammar rather than a 100 percent compatible parser, so provenance, rebuild steps, parse health and the remaining known gaps live in docs/grammars/tree-sitter-cds.md. tree-sitter-wasms does not ship it.

Extraction (real repos)

Repo	Size	Files	.cds	Nodes	Edges	verify-extraction
SAP-samples/fiori-elements-feature-showcase	Small	63	19	320	425	PASS
SAP-samples/btp-cap-multitenant-saas	Medium	144	27	507	1,015	PASS
gregorwolf/bookshop-demo	Large	766	74	7,011	14,160	PASS

Unit tests: CDS block in __tests__/extraction.test.ts plus __tests__/cds-resolution.test.ts (scoped lookup, alias FQN, self-exclusion, ambiguity, import paths) and __tests__/cds-cross-file.test.ts (end-to-end db/srv/app project).

Retrieval A/B

Two with/without runs (scripts/agent-eval/run-all.sh, Sonnet, effort high, n=1 per arm), one per corpus question on the Small and Medium repos:

Repo	Arm	duration	tool calls	Read	codegraph	cost
fiori-elements-feature-showcase	with	43s	7	3	0	$0.31
fiori-elements-feature-showcase	without	25s	4	3	0	$0.17
btp-cap-multitenant-saas	with	20s	3	1	0	$0.18
btp-cap-multitenant-saas	without	22s	4	2	0	$0.16

Both arms reached the right files and the right answer every time. The with-arm never called codegraph: the MCP server attached (1 tool exposed) but Sonnet grepped LROPODataService / AdminService and read the two or three .cds files that hold the whole answer. CDS models are small and grep-friendly, so a retrieval win is not the merge bar here; extraction quality (the table above, the cross-file edges, and the three flow probes below) is.

Flow probes over the index (references / implements edges, up to 4 hops): fiori LROPODataService::RootEntities -> sap.fe.showcase::RootEntities -> (includes) rootBasis -> (criticality) -> sap.common::Criticality connects; btp AdminService::Members -> susaas::db::Members -> (user) -> susaas::db::Users -> (role) -> susaas::db::Roles connects (context susaas.db is stored as scope susaas plus db; dot-normalized it is the CDS name); bookshop-demo AdminService::Books -> my.bookshop::Books -> (genre) -> my.bookshop::Genres connects (its managed and sap.common.CodeList includes come from @sap/cds/common and stay unresolved by design).

Follow-ups (not in this PR)

• select-list columns of projections and views as nodes (renamed and calculated fields)
• gen/ build output of cds build is not ignored by default (it can duplicate .cds sources)
• Fiori @UI annotation semantics and srv/*.js handler wiring (srv.on('READ', 'Books', ...)) as cross-language edges
• WHEN / branch-guard rules do not apply (declarative language)

Re-index CDS projects after upgrading.