fix(explore): a symbol written with its package resolves, like refresh.Discovery.refresh (#2750)

Explore's token resolution, codegraph_node's lookup and the callers,
callees and impact lookups dropped every Go symbol written the way Go
code names it: `refresh.Discovery.refresh`, `storage.Queryable.Querier`
and `gin.Context.Next` resolved to nothing, and so did `binding.Default`.
Three gaps:

- A qualified name was matched against the qualifiedName, which holds
  the type and not the package, or entirely against the path, which
  holds the package and not the type, never as a package followed by a
  type. A new matcher stage reads `pkg.Type.method` as leading
  qualifiers that name the package by path segment and trailing ones
  that end the node's scope. A Python `pkg.mod.Class.method` and a Rust
  `module::Type::method` read the same way.
- A module's root package has no directory: gin's `context.go` is
  `package gin`. A Go package qualifier now also matches the package the
  file declares, read from its `package` clause (src/graph/go-package.ts).
  That covers versioned `v2/` directories and generated packages like
  prometheus's `io_prometheus_client` as well. The clause is read last
  and only where it can decide, a top-level function or a node whose
  scope already ends with the written type, so a common name's hundreds
  of definitions cost no file reads.
- Explore and codegraph_node took a qualified token's matches from the
  full-text search's top 50 only, and for gin's `binding.Default` those
  were three `binding/default_validator*.go` files. Through one shared
  helper they now fall back to the exact-name index when the search has
  no match; the CLI's lookup already started there.

The new stage runs only after the existing ones fail, and the search
keeps its order, so a name that resolved before keeps its result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/bin/codegraph.ts

@@ -72,6 +72,7 @@ import { getTelemetry, TELEMETRY_DOCS, recordIndexEvent } from '../telemetry';
 import { BROWSER_ENV, DEFAULT_UI_PORT } from '../ui-server/constants';
 import type { UiServerHandle } from '../ui-server';
 import { lookupSymbolNodes, describeSymbolNode, groupDefinitions } from '../graph/symbol-lookup';
+import { goPackageReaderFor } from '../graph/go-package';
 import type { Node, Edge } from '../types';
 import { isTestPath } from '../search/query-utils';
 
@@ -2332,7 +2333,7 @@ for (const direction of ['callers', 'callees'] as const) {
         const cg = await CodeGraph.open(projectPath);
         try {
           const limit = parseInt(options.limit || '20', 10);
-          const { nodes: targets } = lookupSymbolNodes(cg, symbol);
+          const { nodes: targets } = lookupSymbolNodes(cg, symbol, goPackageReaderFor(cg));
           if (targets.length === 0) {
             info(formatSymbolNotFound(symbol, cg.searchNodes(symbol, { limit: 5 }).map((m) => m.node.name)));
             return;
@@ -2456,7 +2457,7 @@ program
       const cg = await CodeGraph.open(projectPath);
       try {
         const depth = Math.min(Math.max(parseInt(options.depth || '2', 10), 1), 10);
-        const { nodes: targets } = lookupSymbolNodes(cg, symbol);
+        const { nodes: targets } = lookupSymbolNodes(cg, symbol, goPackageReaderFor(cg));
         if (targets.length === 0) {
           info(formatSymbolNotFound(symbol, cg.searchNodes(symbol, { limit: 5 }).map((m) => m.node.name)));
           return;