PR: merge deterministic math milestone (M2.0 Scalar Foundation)

[flyingrobots]

This issue exists solely to satisfy the repo linkage policy for the PR that merges the deterministic-math milestone work currently on F32Scalar/sin-cos.

Scope

• Open a PR from the branch tip to main.
• Ensure PR body includes Closes #<this-issue-number>.
• Follow the CodeRabbit review loop; do not merge via admin bypass.

Major deliverables in the branch

• Deterministic F32Scalar trig via LUT + guardrails.
• det_fixed backend lane (DFix64) + CI lanes (glibc/musl/macOS).
• Motion payload v2: Q32.32 (6×i64 LE) with dual decode (v0/v2) and executor ported to Scalar.

CodeRabbit workflow reminder

After CodeRabbit posts review comments, follow /Users/james/git/jitos/docs/procedures/EXTRACT-PR-COMMENTS.md to extract actionable comments, batch-fix, and iterate until approvals.

[flyingrobots]

PR #167 is updated, mergeable, and CI is green. If you merge it, this issue will auto-close via 'Closes #165' in the PR body.