feat(crossrepo): bounded cross-repo base-lib symbol linking (depth-1, capped, default-off)

Add selective cross-repo base-library symbol linking so an unresolved
callee (NOT in the local repo index) can pull the called base-lib function
DEFINITION in as the deepest dependency chunk, tagged dep_source='crosslib:<repo>'.

(1) scripts/crossrepo/build_global_symbol_index.py — stream-extracts selected
    base-lib subtrees from the corpus tarball in ONE pass and indexes PUBLIC
    function+type DEFINITIONS via libclang into a resumable SQLite store. A1
    (boost/abseil/folly/openssl/boringssl/protobuf/eigen/fmt/glib) full public
    API; A2 (std/libc) public-symbol-only with reserved-name + trivial-body
    gating. Fail-loud: missing tarball / libclang load / bad subtree / 0-extract
    RAISE; per-file 3rd-party header parse errors are counted+skipped.

(2) index_project.collect_transitive_deps — when --global-symbol-index is set,
    an unresolved callee is looked up in the global store and recorded as a
    depth-1 cross-lib pull under HARD BOUNDS (CrossLinkBudget: cap 12 deps/doc,
    6144 tok/doc); pulled defs are LEAVES (their base-lib-internal callees are
    never followed). build_training_documents emits them as the deepest dep
    chunks (6-tuple PartInfo carrying dep_source), resolves root->base-lib
    call_edges to the pulled chunk, and flows through the SAME dep_levels/topo/
    materialize/pack path. DEFAULT off -> behavior byte-identical when absent.

(3) --global-symbol-index threaded through streaming_conveyor.py and
    streaming_reindex.py (CLI -> fail-loud check -> process_one_repo ->
    index_project), all trailing-default params (running conveyor unaffected).

Tests: tests/test_crossrepo_global_symbols.py (5/5) — real SQLite store
round-trip, public-symbol gating, budget bounds, off=unchanged, on=tagged
chunk + resolved root->crosslib call_edge. Gitignore the 116 MB build store.

Author: apstenku123

.gitignore

@@ -81,3 +81,7 @@ outputs/packed/1024/build_summary.json
 outputs/packed/1024/build.log
 outputs/verification_report/_summary.json
 outputs/verification_report/_samples/_manifest.json
+outputs/crossrepo/global_symbols.sqlite
+outputs/crossrepo/global_symbols.sqlite-shm
+outputs/crossrepo/global_symbols.sqlite-wal
+outputs/reindexed_commits/*