docs: deployment solver implementation plan (v0.3.0 scope)

9 tasks: spar-solver crate, topology graph, constraint extraction,
FFD/BFD bin-packing, virtual bus library (13 protocols), source
rewriting via rowan, spar allocate CLI, impact preview, integration.

Addresses STPA requirements: SOLVER-REQ-020 (no silent defaults),
SOLVER-REQ-023 (deterministic), SOLVER-REQ-016 (parse-after-rewrite).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: avrabe