Execute structured Lean imports in isolated REPLs

[Deicyde]

Summary

• add an optional ordered imports parameter to run_lean_code
• resolve and revalidate project modules under the request deadline before execution
• run each nonempty structured-import request in a fresh bounded REPL process and retire it afterward
• reject mixed or ambiguous source import headers before discovery or process use
• avoid retries after dispatch and remove process-local environment and proof-state handles from responses
• preserve the existing path for omitted, null, and empty imports

Depends on #30. The reviewable change is commit 5eb3bc1; earlier commits belong to the dependency stack. This restack preserves the original patch exactly while inheriting #30's repaired response boundary.

Validation

• range-diff and stable patch ID: unchanged from fe54b7b
• Python 3.10 cumulative full suite: 798 passed, 5 deselected
• Python 3.13 cumulative full suite: 797 passed, 1 skipped, 5 deselected
• focused structured/runtime suite: 293 passed on both Python versions
• independent restack review: 295 focused tests and exact merge-tree SHIP
• real Lean suite: 5 passed
• make lint
• make check-example
• git diff --check