fix(toolchain): print nix attrs before resolving the closed sets (RIG-5489)

[rigel-mintaka]

Summary

tools/toolchain/parity.ts --print-nix-attrs now exits straight after the devenv.nix parse and that parse's own empty-result refusal.

• Before this change it evaluated the closed sets langs, goAnalysis and meissa before printing.
• The pgtest sqlc step and the e2e dev-shell step call it only to get the attribute list. They paid for those evaluations and never used them.
• The full parity check is unchanged. It still refuses an empty closed set before it reports any verdict.

Measured

Dispatch run 38841392253 on this head, compared with 38839237527 on main plus T3:

Step	Before s	After s
e2e: dev shell nixpkgs tools	191	15
pgtest: nixpkgs tools for sqlc vet	125	14

In that run moon (go) failed. The nilaway Go-modules fetch got a transient download error from the Go module proxy storage, outside this diff, so I re-ran the failed jobs.

Tests

tools/toolchain/parity.test.ts:

• New: --print-nix-attrs with every closed-set JSON missing from the stub nix prints ["buf"] and exits 0. Before the fix it failed (red run: 11 pass, 1 fail).
• New: an empty devenv.nix parse still exits 1.
• New: an empty langs set fails the verdict path. It fails when that guard is removed (12 pass, 1 fail).
• bun test tools/toolchain/: 117 pass, 0 fail. biome ci tools/toolchain: clean apart from 2 warnings that already exist on main.

Design: docs/designs/compass/ci-pr-speed.md § T2 in RigelBuild/docs. Issue: RIG-5489.

[trunk-io]

😎 This pull request was merged.

[linear-code]

RIG-5489

[github-actions]

Compass engineering docs preview: https://ci-rig-5489-parity-print-att.compass-eng-docs.pages.dev

Deployed from ci/rig-5489-parity-print-attrs at 9c2d13d.

[rigel-mintaka]

Review loop on head 9c2d13d5 (base main 888ac07e).

• Round 1 at 52ec47fa: one medium. No test covered the empty-langs refusal I rewrote. Fixed in 9c2d13d5, and checked by removing the guard: the new test fails.
• Now: high 0, medium 0, low 0.

[rigel-mintaka]

Review loop on head 9c2d13d5: round 2 is high 0, medium 0, low 0 (the round-1 medium was fixed in 9c2d13d5). Dispatch 38841392253 on this head is green after re-running the failed jobs.