Add KB scale stress tests and launch mode docs

docs/kb_scale_validation.md

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+# KB Scale Validation
+
+Status: Added on `beta` (March 1, 2026).
+
+This document defines stress-oriented validation for knowledge-base behavior
+under large index counts.
+
+## Coverage
+
+1. Sort/tag/search over large indexes:
+  - `src/tests/test_kb_scale_stress.py::test_kb_sort_tag_search_scale`
+2. High-degree graph link handling:
+  - `src/tests/test_kb_scale_stress.py::test_kb_graph_high_degree_links_scale`
+3. Textual TUI v2 large-index interactions:
+  - `src/tests/test_tui_v2_kb_scale_stress.py::test_tui2_kb_large_index_navigation`
+
+## Dataset Profiles
+
+- Standard:
+  - `10,000` entries for sort/search/tag
+  - `1,000` graph edges from one source node
+  - `10,000` TUI rows
+- Stress (`--stress`):
+  - `100,000` entries for sort/search/tag
+  - `5,000` graph edges from one source node
+  - `50,000` TUI rows
+
+## Commands
+
+Standard run:
+
+```bash
+.venv/bin/python -m pytest -q src/tests/test_kb_scale_stress.py src/tests/test_tui_v2_kb_scale_stress.py
+```
+
+Stress run:
+
+```bash
+.venv/bin/python -m pytest -q --stress src/tests/test_kb_scale_stress.py src/tests/test_tui_v2_kb_scale_stress.py
+```
+
+## Notes
+
+- Stress scenarios are opt-in and require `--stress`.
+- Time budgets are intentionally conservative to catch regressions while
+  reducing CI flakiness across environments.