docs(m183): a workaround the Zig needed is not a design Rust keeps

Scopes the audit of ported Zig workarounds across rustd/. Orders the
five slices by what a wrong answer costs rather than by which grep found
them: the connection-string parser first, because a Postgres password
containing '?sslmode=' currently makes the daemon skip the TLS upgrade
it means to impose.

The diagnostic is sharpened from 'is there a crate for this' to the
three-stage split config/raw/mod.rs already documents — serde does
shape, garde does bounds, we do meaning. That question predicts all
four M177 findings and every deliberate keep; the crate question does
not, because it answers wrong for afd_crypto::secret's length check.

Adds a fourth verdict, UNIFY, for the concept spelled N times with N
policies, and makes the undocumented keeps a first-class deliverable —
the ~15 hand-rolls M177 judged deliberate left no record, so the next
auditor re-derives them.

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