chore: pin numpy<2.0 and align plan with tech-audit research

Research doc (2026-04-18 OSS stack audit) flagged NumPy 2.0 as silently
breaking GTSAM Python bindings (issue #2264). Pin numpy>=1.26,<2.0 and
constrain opencv-python-headless<4.11 (knock-on: 4.11+ requires numpy≥2).

Verified after downgrade:
  - 196 passed / 8 skipped unit/component
  - EuRoC MH_01 e2e PASS (no regression on 0.205m ESKF ATE baseline)

Plan updates in next_steps.md §5:
  - cuVSLAM strategy clarified: Mono-Depth (barometer as synthetic depth),
    not Mono-Inertial (needs stereo hardware we don't have)
  - DINOv2-VLAD (AnyLoc) for GPR + FP16 TRT (INT8 broken for ViT on Jetson)
  - GTSAM: documented that 4.2 stable is not on PyPI (only 4.3a0), so
    deferred to post-sprint-1 ESKF-only path stays the right call
  - VPAIR xfail root cause: no raw IMU + mock satellite index (verified
    with scale=1.0 and scale=45.0 runs — ATE stays at ~1236m ESKF /
    ~1770km GPS regardless of scale)
  - Flight controller H743 vs F405 check flagged as critical blocker

README "next steps" section rewritten to match the research-aligned plan.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -15,8 +15,8 @@ dependencies = [
     "python-multipart>=0.0.9",
     "httpx>=0.27",
     "diskcache>=5.6",
-    "numpy>=1.26",
-    "opencv-python-headless>=4.9",
+    "numpy>=1.26,<2.0",  # NumPy 2.0 silently breaks GTSAM Python bindings (issue #2264)
+    "opencv-python-headless>=4.9,<4.11",  # 4.11+ requires numpy>=2.0 (incompatible with GTSAM)
     "gtsam>=4.3a0",
     "pymavlink>=2.4",
 ]