chore(m175): open workstream 001 — Rust workspace scaffold

CHORE(open) for M175_001. Spec moves pending/ -> active/, Status becomes
IN_PROGRESS, Branch is recorded, and the Test Baseline is taken from the
repository's Zig test counter (make _lint_zig_test_depth): unit=4237
integration=719. The Rust cargo suite starts empty — this milestone creates
it — so it is tracked separately by cargo test --workspace.

The PR Intent handshake is filled at the same time: the Intent restated in
the implementing agent's own words plus nine assumptions, the load-bearing
ones being that fixtures flow only from Zig to Rust, that the fixture
emitter compiles under zig run without touching build.zig, and that neither
crate pulls an async runtime (Invariant 2).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>