fix(code-index): disclose ambiguous calls as caller gaps

[devin-ai-integration]

Summary

• Ambiguous call references (more than one candidate the resolver cannot choose between) now become disclosed unresolved-call gaps instead of disappearing, so callers/callees/impact answers report themselves as partial.

Motivation

Fixes #3012. resolve_cross_file_reference returned None when more than one inherent candidate remained, and same-file resolution in resolve_file_references dropped ambiguous references (_ => {}). Neither path left a gap row, so an incomplete radius looked complete.

Changes

• production/helpers.rs: resolve_cross_file_reference returns ReferenceResolutionV1::{Bound, Ambiguous}; per-file resolution collects ambiguous Calls references as gaps. CrossFileResolutionV1.implementor_gaps is renamed to gaps and carries these rows too; resolve_selected_cross_file_references returns the full resolution.
• production/sparse_resolution.rs: the sparse (incremental) path adds those gaps to the owning file's unresolved_calls, so incremental and full seals agree.
• chunks.rs / chunks/artifacts.rs: deleted the same-file ambiguity special case. Ambiguous references are now retained with a new CodeIndexUnresolvedReferenceV1.ambiguous_local flag (serde-skipped when false, so existing artifacts are unchanged). Sealing treats a flagged reference as Ambiguous before any import lookup, so it is always a gap and can never bind a shadowed import. Java overloads keep their existing path.
• Unit test qualified_reference_selects_its_namespace_and_bare_name_stays_ambiguous now asserts the ambiguous bare name is retained, not dropped.

Choice made: ambiguous candidates are disclosed as gaps, not emitted as low-confidence edges. The issue proposed either option; gaps add no false edges to graph answers.

Test plan

• relation_coverage::a_type_path_call_with_two_inherent_candidates_makes_both_callers_partial (cross-file Clock::tick() with cfg-gated impls in two files): fails on master, passes with the fix.
• relation_coverage::a_same_file_call_with_two_candidates_makes_both_callers_partial (Ruby) and a_same_file_python_call_with_two_candidates_makes_its_callees_partial (Python): fail without the same-file change, pass with it.
• relation_coverage::locally_ambiguous_definitions_shadow_an_imported_name: no false caller on the imported pkg.lib.scale. Fails on the first commit, passes now.
• cargo test -p tracedecay-code-index: all pass (285 + 209 + 4 + 3)
• cargo clippy -p tracedecay-code-index --all-targets -- -D warnings: clean
• cargo fmt --all -- --check: clean
• Full workspace cargo nextest run left to CI.

Checklist

• CHANGELOG.md updated (release-please manages it)
• No secrets, credentials, or .env files included
• Breaking changes documented (if any): none

Link to Devin session: https://app.devin.ai/sessions/fd546572dcdb49cbbf8f10b6dec5ec80
Open in Devin Desktop: https://app.devin.ai/desktop/session/fd546572dcdb49cbbf8f10b6dec5ec80?variant=devin
Requested by: @ScriptedAlchemy

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[devin-ai-integration]

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