Phase 2 — precision-aware contraction path selection

[scttfrdmn]

Roadmap phase tracker

This issue tracks trntensor's work on Phase 2 of the trnsci roadmap.

See the suite-level roadmap
for the full phase matrix and cross-project dependencies. A reader-friendly
version lives at trnsci.dev/roadmap/.

What this phase means for trntensor:
Some contractions accumulate long chains (DF-MP2 energy, THC / CP
rank-sweeps, tensor network contractions). Phase 2 teaches the
contraction planner to pick precision-appropriate paths — FP32 for
non-critical, double-double for chemistry, Kahan-compensated for
long-chain reductions.

Done means:

• plan_contraction accepts a precision="fast"|"kahan"|"dd" knob.
• precision="dd" routes through trnblas double-double GEMM (Phase 2 dependency).
• Precision-aware benchmarks: same contraction, three precisions, three numbers.
• Documented in docs/architecture.md with the CP / Tucker + THC use cases.

Coordination

• Label: phase-2-precision — matches the same label in every sub-project.
• Cross-project dependencies, if any, are called out in the "Done means"
  list above. Link any PRs / child issues here for tracking.

Close this issue when all "Done means" items are satisfied.