feat(xtask): add ci plan subcommand (Rust-native PR planner)

[EffortlessSteven]

Summary

Replaces the inline Python planner in .github/workflows/pr-plan.yml (added in #3742) with a Rust-native, unit-testable xtask ci plan subcommand. This is PR K in the multi-PR CI roadmap (A–H merged; I, J, K open).

cargo run --locked -p xtask -- ci plan \
  --base "$BASE_SHA" \
  --head "$HEAD_SHA" \
  --labels-json '["full-ci"]' \
  --json-out ci-plan.json \
  --github-summary "$GITHUB_STEP_SUMMARY"

Why

The inline Python version was the right first move (#3742 shipped fast and exercised the LEM model end-to-end) but it could not be tested without a CI run. A YAML-embedded planner drifts; a Rust planner can be tested with fixtures and held to the same review bar as the rest of the codebase. This is the foundation for everything that follows: PR L (policy files), PR N (actuals), PR O (budget warnings), and PR P (ripr promotion rules) all consume ci-plan.json.

Implementation

• xtask/src/ci_plan.rs — pure plan_for(changed, labels) -> Plan function + git_changed_files(...) wrapper + run(...) entry point. Same touched-area buckets and lane logic as the Python version, plus the ripr lane (rust_production, ripr label).
• xtask/src/main.rs — new Cmd::Ci { command: CiCmd } subcommand and CiCmd::Plan variant.
• .github/workflows/pr-plan.yml — drops ~120 lines of inline Python in favor of a single cargo run invocation. Adds a small Rust toolchain install + Swatinem/rust-cache for the xtask compile.

Schema

Two additions vs the inline Python version:

Field	Type	Purpose
lane.blocking	bool	Whether this lane blocks merges today (placeholder for budget enforcement).
lane.stage	"required" | "default" | "label" | "advisory"	Lane-class taxonomy for PR P (ripr promotion).

The rest of the schema (posture, touched, labels, lanes[].name/lem/reason, estimated_lem, band) is unchanged from #3742.

Tests (10/10 passing)

Test	Asserts
empty PR	0 lanes, pennies band
docs-only PR	guards lane only, posture=docs-only
rust crate change	Core / Feature Matrix PR / MSRV / ripr / guards (54 LEM, elevated band)
GPU kernel change	native compile fires; Docker does not without label
full-ci label	all label-gated lanes fire
ripr-eligible PR	advisory lane fires
test-only Rust diff	ripr advisory does not fire (tests/ ≠ rust_production)
ripr label on docs-only PR	ripr advisory still fires
full-ci on kernel change	>125 LEM, over-hard-ceiling band
parse_git_output	strips blank lines

cargo test --locked --no-default-features -p xtask --bin xtask ci_plan
# test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured

Smoke output

$ cargo run -q -p xtask -- ci plan --dry-run --labels-json '["full-ci"]'
{
  "posture": "empty",
  "touched": { ... },
  "labels": ["full-ci"],
  "lanes": [
    { "name": "BDD Grid Check", "lem": 4, "stage": "label", "blocking": false, ... },
    { "name": "Clippy (macOS ARM64)", "lem": 150, "stage": "label", ... },
    { "name": "Compatibility (ABI/FFI)", "lem": 8, ... },
    { "name": "Compatibility (tokenizer)", "lem": 6, ... },
    { "name": "Property Tests (smoke)", "lem": 4, ... },
    { "name": "ripr static exposure (advisory)", "lem": 4, "stage": "advisory", ... }
  ],
  "estimated_lem": 176,
  "band": "🚨 over hard ceiling (> 125 LEM)"
}

Test plan

• CI Core green on the new xtask code (clippy + tests).
• PR Plan workflow runs the new Rust planner on this PR and posts the same shape of step summary.
• ci-plan.json artifact uploaded with new schema.
• No behavior change for downstream consumers — all existing fields preserved.

Out of scope (future PRs)

• PR L: hoist hard-coded LEM estimates and lane definitions into policy/ci-budget.toml, policy/ci-lanes.toml, policy/ci-risk-packs.toml.
• PR M: nextest profile + JUnit upload + per-test duration history.
• PR N: actual LEM collection (ci-actuals.json).
• PR O: soft budget warnings (warn at 35 / 75 LEM; hard at 125 without full-ci).
• PR P: ripr promotion rules using lane.stage taxonomy added here.

https://claude.ai/code/session_01S2yTnEYJcA3G2CyZn9bY4v

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