test(design-import): isolate network-asset and knobclean cases from shared state

[shipyard-local]

Harden the two design-import test-isolation weaknesses behind the rare
full-run flake where "DesignIR asset manifest reports missing fetcher
and empty network downloads" misses its asset-empty diagnostic
(first seen on the Shipyard mac lane 2026-05-21, mitigated then by
timeout bumps + the design-import-network ctest resource lock):

• Unique per-invocation asset URLs in the missing-fetcher/empty-download
  case. The on-disk asset cache is keyed by absolute URL, so fixed URLs
  rely on every test and every concurrent process keeping a private
  cache_directory to avoid a cache hit that would skip the download and
  its diagnostic. Deriving the URL path from the unique TempDir name
  makes that collision structurally impossible.
• Align network_timeout_ms with the other two network smoke cases
  (30s): the fetch path reclassifies a slow child spawn as
  asset-fetch-timeout, replacing the asserted diagnostic under co-load.
  The fake curl never sleeps, so the wide timeout adds no wall time.
• Capture the diagnostic codes actually present (new diagnostic_codes
  helper) so a future flake reports which diagnostic replaced the
  expected one instead of just "missed asset-empty".
• Give the knobclean durable-cache case a unique per-invocation home
  directory: the fixed cwd/knobclean-home name is shared state between
  concurrent runs of the binary — one process's remove_all races
  another's fs::exists(emitted) check (reproduced under 8-way
  concurrent full-suite runs; 48/48 green after the fix).

No production code changes; observable download/cache behavior for
real imports is untouched. Full suite green in declaration order and
across 10 randomized seeds with identical counts (2556 assertions in
367 test cases).

Hotspot-Grow: test/test_design_import_codegen.cpp reason="isolation comments + diagnostics capture on the flake-prone network case"
Hotspot-Grow: test/test_design_import_ir.cpp reason="unique per-invocation knobclean home dir + rationale comment"
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_014e8kbMn8Sf4mF8ozsy1Jot

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🔎 Provenance

Agent	claude
Machine	m5
Workspace	w1
Directory	~/Code/fig-fix
Session	fd0e374f-d5dc-4e2a-bb33-6fa8b66124a0

Resume

claude --resume fd0e374f-d5dc-4e2a-bb33-6fa8b66124a0

Jump to this tab

cmux surface focus B1B434D3-85F6-4937-8806-96D59886F574

Relaunch (any agent)

cmux surface resume get --surface B1B434D3-85F6-4937-8806-96D59886F574

Restore URL — https://claude.ai/code/session_014e8kbMn8Sf4mF8ozsy1Jot

_{stamped 2026-07-19 17:36 UTC}

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Summary by cubic

Hardened design-import tests to eliminate a rare CI flake by isolating shared state, aligning network timeouts, and improving diagnostics. No production code changes.

• Bug Fixes
  • Use unique per-run asset URLs to prevent cache hits across tests/processes.
  • Set network_timeout_ms to 30s to match other network smoke tests and avoid misclassified timeouts.
  • Add diagnostic_codes helper to surface actual diagnostics when assertions fail.
  • Give the knobclean durable-cache test a unique per-run home directory to avoid concurrent fs races.

^{Written for commit 4dc54d1. Summary will update on new commits.}