feat(extraction): capture function-as-value — callback registration sites in callers/impact (#756) (#807)

A function name used as a VALUE — passed as an argument
(signal(SIGINT, handler), qsort(..., compare)), assigned to a function
pointer or field (ops->recv_cb = my_cb, OnClick := Handler), or placed in
a struct initializer / handler table ({ .recv_cb = my_cb },
{ "get", getCommand }) — produced no edge in ANY of the 19 tree-sitter
languages, so registered callbacks looked dead and their registration
sites were invisible to callers/impact.

This adds table-driven function-as-value capture across all 19 languages
(plus the wrapper forms: &fn, &Cls::method, Java Class::m, Kotlin ::f,
Swift #selector, ObjC @selector, Ruby method(:sym), Scala eta, Pascal
@Handler), gated at extraction (same-file definitions + imported
bindings; C-family file-scope initializers are constant-expression
contexts and skip the gate, which is how redis-style cross-file command
tables resolve), and resolved by a dedicated strategy: function/method
targets only, same-file first, unique-or-drop cross-file, no fuzzy
fallback ever. Edges persist as kind 'references' with metadata.fnRef,
so getCallers/getImpactRadius surface them with zero graph-layer
changes; MCP callers/callees label them "via callback registration".

Precision rules bought by real-repo false positives (full A/B record in
docs/design/function-ref-capture.md): C++ is &-explicit outside
file-scope tables (fmt's begin/out/size collisions; out-of-line member
defs are function-kind); TS/JS/Python bare ids resolve to functions only
(TS class fields extract as method-kind — pre-existing quirk); Swift
refuses same-file method overload-families; param-forward shapes
(this.x = x, value: value) and destructuring are skipped; minified
bundles (*.min.js) produce no candidates.

Validated on 17 public OSS repos (redis, excalidraw, gin, bytes, okhttp,
okio, Alamofire, flask, sinatra, Newtonsoft.Json, scopt, provider,
busted, Fusion, AFNetworking, PascalCoin, fmt): node counts identical,
zero calls edges lost or gained, references strictly additive
(+3,200 registration edges total), precision spot-checked by reading
sampled source lines (redis 30/30, flask 8/8). Deliberately NOT covered:
indirect-dispatch resolution (o->cb(x) → impl) — that needs data-flow
through struct fields, and a wrong edge is worse than none.

EXTRACTION_VERSION 18 → 19 (re-index to benefit).

Closes #756

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### New Features
 
+- CodeGraph now sees where a function is **registered as a callback**, not just where it's called. A function name passed as an argument (`signal(SIGINT, handler)`, `qsort(…, compare)`, `addEventListener(…, onBlur)`), assigned to a function pointer or field (`ops->recv_cb = my_cb`, `OnClick := Handler`), or placed in a struct initializer or handler table (`{ .recv_cb = my_cb }`, `{ "get", getCommand }`) now produces a reference edge from the registration site to the function — so `codegraph_callers` and `codegraph_impact` surface callback wiring that previously looked like dead code. Works across all supported languages, including the language-specific forms: C/C++ `&fn`, Java `Class::method`, Kotlin `::fn`, Swift `#selector`, Objective-C `@selector`, Ruby `method(:fn)`, Scala eta-expansion, and Delphi/Pascal `@Handler` and `OnClick := Handler` event wiring. Callers output labels these "via callback registration". Resolution is deliberately conservative: an ambiguous name produces no edge rather than a wrong one. Re-index a project to benefit. Thanks @zmcrazy. (#756)
 - The `codegraph_node` MCP tool can now **read a whole source file like the built-in Read tool — only faster, served from the index**. Pass a file path with no symbol and it returns that file's current source with line numbers (the same `<n>⇥<line>` shape Read produces, so an assistant can edit straight from it), narrowable with `offset`/`limit` exactly like Read, plus a one-line note of which files depend on it (the file's blast radius). Use it anywhere you'd reach for Read on an indexed source file. Pass `symbolsOnly: true` for just the file's structure. Configuration/data files (`.yml` / `.properties`) are summarized by key only, never dumped, so secrets in them are never surfaced. The agent-facing guidance was also retuned so assistants reach for codegraph while *implementing* a change (not only when answering questions), since one codegraph call returns the same bytes plus the blast radius, faster than re-reading the file.
 - New `codegraph upgrade` command updates CodeGraph to the latest release in place — it detects how you installed (the standalone `install.sh` / `install.ps1` bundle, npm, or npx) and does the right thing for each, on macOS, Linux, and Windows. Use `codegraph upgrade --check` to see whether an update is available without installing, or `codegraph upgrade <version>` to move to a specific version. After upgrading it reminds you to re-index your projects so they pick up the newer engine's improvements. (#679)
 - `codegraph status` now flags when a project's index was built by an older engine than the one you're running and recommends re-indexing (also surfaced in `codegraph status --json`), so you know when a `codegraph index -f` or `codegraph sync` will add coverage a newer release introduced.