feat(rustd): cargo workspace and afd_core domain primitives

Sections 1 and 2. The workspace roots at rustd/ as a virtual manifest with an
explicit member list and every crate a sibling under crates/ — Microsoft's
M-CRATES-FLAT-FOLDER, chosen by Indy over bun's src/<name> nesting on Aug 23,
2026; bun still supplies the mechanics (virtual root, workspace dependencies,
the warnings-are-errors lint rationale). Toolchain pinned to 1.98.0, matching
the mise-managed Rust so a shell and a container cannot resolve different
compilers.

afd_core holds three primitives, each a newtype whose invariant is checked once
at the boundary:

Uuid7 ports src/agentsfleetd/types/id_format.zig, which REJECTS uppercase
rather than normalizing it — Postgres folds ::uuid to lowercase, so an
uppercase spelling is the same row there and a different key in every
text-keyed store. The uuid crate cannot express that (it parses
case-insensitively), so identity is a validating newtype and the crate keeps
one less dependency. Deserialization goes through String rather than a borrowed
str on purpose: a JSON producer may escape any character, and an
escaped-but-canonical identifier has to be accepted exactly as the Zig parser
accepts it — rejecting it would be a behaviour difference dressed as an
optimization.

ErrorCode is a checked SUBSET of the Zig registry, never a second registry.
Its grammar is validated during constant evaluation, so a malformed code is a
build failure, and a test reads error_registry.zig and fails if a code here is
absent or spelled differently there.

WorkerCount carries both paths deliberately: clamping() for the assignment
path, because that is what the Zig daemon does and what RegisterResponse echoes
back, and new() for an operator surface that can be told it is wrong.

Twenty-three tests, all through the public surface. Two proven to bite by
seeding the defect they exist to catch: adding tokio to afd_wire fails
test_core_dependency_freeze, and removing a member's lint inheritance fails
test_workspace_lint_policy.

Dimensions 1.1, 1.2, 2.1, 2.2 and 2.3 marked DONE. Dimension 1.2 is amended in
the same commit: it specified asserting the lint set from cargo metadata, which
does not expose lints under cargo 1.98 — the assertion reads the workspace
manifest instead and additionally proves every member inherits the policy,
which is the failure metadata could not have caught either.

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