fix(visualizer): bounded incremental file discovery + fix test gRPC hang

[SushantGautam]

Problem

The visualizer file-tree scan (GET /api/visualizer/api/files/) performed an unbounded os.walk over the results directory, parsing every JSON file with no limits. When --results_dir resolved to a large tree (e.g. $HOME via uvx simpleaudit-studio@latest --visualize-only --results_dir ./), the scan took minutes and the browser fetch stayed pending forever ("Loading files...").

Additionally, the full infra/tests suite consistently hung at test_api_e2e.py::APIE2ETest::test_cancel_queued_run because the test triggered a real gRPC call to the unresolvable hatchet-server:7077 (compose default). After ~50 prior tests the gRPC C-core thread pool was exhausted, so the call blocked indefinitely.

Fix

Bounded scan (infra/visualizer.py)

• Limits: 5,000 inspected JSON files, 5s budget, depth 8, 100 MB/file — enforced during traversal, not afterward.
• Partial results with truncated, reason, inspected, elapsed_seconds metadata; never reports a truncated tree as complete.
• Persistent metadata index (~/.simpleaudit-studio/visualizer-index.json) keyed by (path, mtime, size) — unchanged files are not re-parsed.
• watchfiles for incremental updates via a new /api/visualizer/api/files/changed/ endpoint.
• Frontend (static/visualizer.html) shows a truncation warning banner and polls the changed endpoint.
• CLI (simpleaudit_studio/cli.py) warns when results dir is home/cwd and displays scan limits.

Test fix (infra/tests/test_api_e2e.py)

• Mock audits.views.submit_audit_run in setUp to prevent a real gRPC call. Matches the pattern used by test_monitors and test_experiments.

Tests

• 14 new visualizer tests (file/time/depth limits, symlinks, oversized files, valid results beyond cap, metadata index, watchfiles)
• Full suite: 995 passed, 2 skipped, 4 deselected
• Ruff clean

Benchmark

• Real audit-results dir: 0.188s
• Home dir: 5.0s truncated (was: minutes)

Related: #18