Convert to a workspace, and make fault injection opt-in

[eduralph]

Phase 1 of the parity-harness plan: mechanical restructure, no behaviour change. Unblocks the runners.

Why

The repo is a client parity harness now, not a wyrd-specific gate, and the layout should say so. The M4 gate stops being the whole crate and becomes one suite among the ones to come; the fixtures it happened to own are split out so a future differential runner can use them without first agreeing to a MetadataStore.

crates/harness      client-agnostic: $PD_ADDRS, PD's region layout as ground truth,
                    deterministic byte corpora. Nothing here knows what a MetadataStore is.
crates/wyrd-gate    the M4 evaluation gate: the vendored contract, the TikvMetadataStore
                    mapping, and the 18 gate tests.

git mv throughout, so history follows (GitHub shows the files as renames). The crate rename client_rust_test → wyrd_gate is the only source change. Same tests, same verdict.

Fault injection is now opt-in — and that is not cosmetic

fail was an unconditional dev-dependency carrying features = ["failpoints"]. In a single package that was merely untidy. In a workspace it is a hazard: cargo test --workspace unifies fail/failpoints into the same tikv-client that a differential runner links — compiling fault-injection machinery into the binary whose entire job is to observe unmodified client behaviour.

Compiled-in failpoints are inert until configured, so this is contamination rather than a live bug. Which is exactly the kind of thing that quietly makes a result untrustworthy months later, when nobody remembers why it's there.

It's now an optional dependency behind a failpoints feature, off by default. (A dev-dependency cannot be optional — Cargo features can't enable dev-deps — so fail moves to [dependencies], the same shape client-rust itself uses.) The failpoint suite declares required-features = ["failpoints"] so it cannot even build without it, and gate-verdict.sh builds only that binary with the feature: the gate suite observes an unmodified client.

Evidence

$ cargo tree -e features -i fail                                 # default build
  failpoints features: 0

$ cargo tree -e features -i fail --features wyrd-gate/failpoints
  failpoints features: 4

$ cargo check -p wyrd-gate --test failpoint_gate --features integration-tests
  error: target `failpoint_gate` in package `wyrd-gate` requires the features: `failpoints`

Full make verdict against a real cluster is unchanged: 17 green, d6 XFAIL on its own assertion, d7 XPASS locally (the sibling checkout carries PR 547's fix; CI runs at the pin, where it XFAILs).

Bonus: the region-encoding tests now run as cluster-free lib unit tests, where before they only ran inside the integration binary.

Also updated for the move: CODEOWNERS, README layout/paths, and the one findings doc that cited a test path.

Checklist

• make check green (workspace-wide clippy -D warnings)
• Verified against a real cluster — verdict identical to pre-conversion
• Cross-vendor reviewed (Codex): no findings
• History preserved (git mv; renames detected)

🤖 Generated with Claude Code

[eduralph]

@codex review