feat(sdk): per-component invariant self-checks + known-answer tests

[Ravenwater]

Add input/output invariant checks and one known-answer test per condition: ImuPreintegrator (ΔR∈SO3 across accumulation; dropped-sample counter for the dt>0 gate); solve_alignment (orthonormal keyframe rotations, finite dR_dbg); init_window/so3_from_matrix (orthonormality of SfM rotations — currently assumed, never checked); two_view (parallax / essential-matrix conditioning / Sampson self-check); pnp (DLT conditioning, reprojection RMS). Acceptance: each precondition is asserted and has a known-answer test that fails when violated. Part of #261. See docs/assessments/vio-diagnostic-methodology.md §C.2.

[Ravenwater]

Triage (2026-06-14) — re-scope: some gates exist, the assertions don't

Already present: two_view has Sampson + cheirality gates. Remaining delta — the explicit invariant assertions + known-answer tests this issue is about:

• init_window / so3_from_matrix orthonormality (init_window.hpp:53 explicitly assumes R is orthonormal, never checks);
• ImuPreintegrator ΔR ∈ SO(3) across accumulation + a dropped-sample counter for the dt>0 gate;
• solve_alignment orthonormal keyframe rotations / finite dR_dbg;
• pnp DLT conditioning + reprojection RMS;
• one KAT per precondition that fails when the invariant is violated.

Priority: P3 (hardening).