v3.19.2 — stale codebase-graph gate

Follow-up to the 3.19.1 ingest deadlock fix. The deadlock guard stopped the
hang; this stops the stale graph it could leave behind.

What it fixes

/cortex-visualize (and ingest_codebase) could serve an out-of-date AST:
the cached code-graph was reused as long as the graph directory merely existed,
with no check that the codebase hadn't changed since it was built. A graph built
before your last edits was reused indefinitely — the "graph shows an old version
of the analysis" report.

The change

New graph_is_fresh(project_path, graph_path): compares the graph artefact's own
build mtime against the newest source file under the project, via a bounded,
ignore-pruned walk that early-exits on the first newer file (vendored/build dirs
— .git, node_modules, target, .venv, search_index, … — are skipped so
they can't trip false staleness). find_cached_graph now skips a stale (or
non-materialised) graph, so ingestion falls through to a fresh analyze_codebase.
It errs fresh on uncertainty (absent project root / unreadable graph) to avoid
needless re-analysis. Scope: ingestion only — the visualizer's roster resolver is
untouched.

Recovering an already-bad cache

If a prior run left a half-built graph (recent, so the freshness gate treats it
as current), force one clean rebuild:

ingest_codebase(project_path="<repo>", force_reindex=true)
# or: rm -rf ~/.cache/cortex/code-graphs/<project>-*/

With the 3.19.1 deadlock guard, that re-ingest completes instead of hanging.

Still in progress

The deadlock guard is a safety net; the architectural root (the pooled MCP reader
bound to a throwaway per-call event loop) and symbol-read producer/consumer
overlap are tracked in tasks/ingest-ecosystem-fix-plan.md.