fix: stash overload siblings instead of discarding them on QN collision

[pranitpanda-affirm]

What does this PR do?

Interface/class methods that share a qualified_name (JDBI @SqlQuery overloads, C++ overloads, Python @overload, etc.) previously had every arrival but one silently destroyed by cbm_gbuf_upsert_node's same-QN tie-break — search_graph/definition lookups would report zero results for a real, present method.

This is Option 1 of two considered fixes. The canonical survivor is picked using the exact same tie-break as before, so CALLS-edge resolution (which is name-only in registry.c and has no way to disambiguate overloads) sees no behavior change — no regression to callers-of enumeration. Instead of discarding the loser, its name, signature, line range, and annotations are folded into a sibling_overloads array on the surviving node's properties_json, so overload existence is discoverable instead of silently vanishing.

Option 2 (symmetric arg-count-aware call resolution, so overload fan-in actually resolves per-overload rather than just being visible) is a larger, separate follow-up — deliberately out of scope here.

Testing

Two new regression tests in tests/test_graph_buffer.c:

• gbuf_upsert_overload_collision_stashes_loser — 2-way and 3-way overload collisions, verifying flattening (not nesting) of stashed siblings.
• gbuf_upsert_same_location_refresh_no_sibling_stash — a same-file/same-line incremental re-emit is not mistaken for an overload collision.

All 52 tests in the graph_buffer suite pass under ASan/UBSan. Applied to the locally-installed binary and smoke-tested against all 30 existing project indexes (no data loss).

Checklist

• Every commit is signed off (git commit -s) — not yet done, flagging for follow-up
• Tests pass locally (make -f Makefile.cbm test) — relevant suites (graph_buffer, store_*, mcp, pipeline) are clean; the full run has ~327 pre-existing failures in tree-sitter grammar/extraction probes, unrelated to this change and present on main before it
• Lint passes (make -f Makefile.cbm lint-ci) — not run locally (cppcheck/clang-format unavailable in this environment); deferring to CI
• New behavior is covered by a test (reproduce-first for bug fixes)