Add indexed call hierarchy navigation to LSP

[Widthdom]

Problem and priority

P2 — posting order 8/10. Audit candidates: D09.

The LSP endpoint supports definitions/references and other navigation, while callers/callees available through CLI/MCP are not exposed as LSP call hierarchy. Adding the standard protocol surface would make recursive caller/callee exploration available directly in editors.

Observed protocol behavior

In a real initialized LSP session using the repository-built server and .cdidx/codeindex.db, definition, references, hover, completion, document symbols and semantic tokens succeeded. Initialize did not advertise call hierarchy; textDocument/prepareCallHierarchy returned:

{"code":-32601,"message":"Method not found: textDocument/prepareCallHierarchy"}

This is a feature request for an unimplemented method, not a failure of an advertised capability. The existing CLI/MCP callers/callees provide the underlying graph navigation.

Implementation instructions and cautions

Start with src/CodeIndex/Lsp/LspServer.Protocol.cs, the existing position-resolution/navigation/location helpers and shared graph query APIs.

• Implement textDocument/prepareCallHierarchy, callHierarchy/incomingCalls and callHierarchy/outgoingCalls, then advertise callHierarchyProvider only when the surface is supported.
• Resolve the selected callable using existing symbol identity and selection rules. Carry bounded opaque item data tied to the indexed generation; do not join by a bare leaf name or silently merge ambiguous symbols.
• Produce protocol-correct UTF-16 range, selectionRange and call-site fromRanges with valid file URIs. Reuse existing LSP coordinate and path policy.
• Keep unresolved/ambiguous references and incomplete graph coverage conservative. Define how the standard protocol reports unavailable/stale/partial data; an empty result must not silently claim compiler-grade completeness.
• Account for unsaved documents, generation changes, cancellation and bounded fan-in/fan-out. Share CLI/MCP graph semantics instead of introducing new language binding or unrelated graph-accuracy changes.

Acceptance and validation

• Use an end-to-end LSP fixture with a caller, callee, overloads/same-name symbols and repeated call sites; prepare/incoming/outgoing results identify correct symbols and ranges.
• Cover unsupported/non-callable positions, incomplete/unresolved graphs, stale items, unsaved changes, Unicode columns, cancellation and high-degree nodes.
• Verify capability discovery and preserve current definition/reference/document-symbol behavior. Document supported languages and the indexed-graph limitations.

Prior history

A targeted closed/open issue search found no exact call-hierarchy request. The audit did not find an advertised call-hierarchy contract that regressed.

Baseline and delivery

Observed during the 2026-09-12 dogfood audit on macOS arm64, using repository-built cdidx 1.49.0 / Debug net8.0 at 36f2e68cb0d7f17bc9591d01ad3045abe870da9b. The build passed with zero warnings/errors. The root database and workspace-manifest freshness checks were healthy, with index and reference-graph completeness true. Counts/timings describe that checkout. Validation during discovery used focused CLI/MCP/LSP reproductions, not the full test suite.

Implement one focused PR for this issue. Follow AGENT_GUIDE.md and the relevant workflows, use the repository-built cdidx for discovery, preserve supported .NET 8/.NET 9 and cross-platform behavior, and avoid new runtime dependencies outside repository policy. Add focused behavioral tests for changed contracts, update affected help/documentation, and include a bilingual changelog.d/unreleased/ fragment for user-visible changes.

[Widthdom]

Work started on branch fix-issue5351 from the freshly fetched origin/main. I will implement indexed LSP call hierarchy for #5351, add focused protocol and graph-safety coverage, update bilingual documentation and a changelog fragment, and run Codex adversarial review before opening the PR.