refactor(gateway): extract interrupt helpers from run.py (slice 19 of #54962)

[andrexibiza]

Related #54962 #55138 #77433

What / Why

Slice 19 of the god-file extraction campaign (#54962): pulls the interrupt-marker helpers out of the 26,823-line gateway/run.py god file into a focused gateway/interrupt_helpers.py module.

Moved (pure move, byte-verbatim, zero behavior change):

• _stamp_hygiene_compression_provenance — best-effort activity provenance stamp for hygiene compression transitions (timeout / cooldown).

Kept in gateway/run.py (documented, not moved):

• _is_fresh_gateway_interruption — depends on _coerce_gateway_timestamp, which is claimed by open PR refactor(gateway): extract display/resolution helpers out of run.py (first slice of #54962) #77433 (display helpers extraction) and may not be moved by this slice. Once refactor(gateway): extract display/resolution helpers out of run.py (first slice of #54962) #77433 lands, the freshness check and its exclusive constant _AUTO_CONTINUE_FRESHNESS_SECS_DEFAULT can follow into this module.
• _AUTO_CONTINUE_FRESHNESS_SECS_DEFAULT — only consumer is _is_fresh_gateway_interruption, which stays.

gateway/run.py re-exports the moved function via a module-attribute import, so gateway.run._stamp_hygiene_compression_provenance keeps resolving with identical object identity (log records also keep the gateway.run logger name).

How to test

python -c "import gateway.run"
python -m pytest tests/gateway/test_restart_resume_pending.py -q --no-header -p no:cacheprovider

Verified in this PR:

• import gateway.run succeeds from the worktree; gateway.run._stamp_hygiene_compression_provenance is gateway.interrupt_helpers._stamp_hygiene_compression_provenance (identity asserted).
• tests/gateway/test_restart_resume_pending.py: 31 passed (the only test file referencing the interrupt cluster).
• git diff --check: clean. scripts/check-windows-footguns.py: no footguns found.
• Moved function is byte-verbatim identical to HEAD:gateway/run.py (diff-checked).

Shrink

gateway/run.py: 26,823 → 26,811 lines (−12). New module: gateway/interrupt_helpers.py (+27). Net line movement is the smallest clean extraction: one function + its private imports; the dependent member stays with its claimed dependency rather than forcing the cluster.

Platforms tested

• Windows 11 (git-bash), CPython via project venv — import + targeted pytest run.
• Pure Python module move; no platform-specific code touched.

Part of #54962

Part of #55138

Part of #78647

[andrexibiza]

At exact head b20be81408a66d9b7b23ad5d78fd39bc118cd43b, commit b20be81408a6 records the interrupt-helper extraction. _stamp_hygiene_compression_provenance now lives in the focused interrupt helper module and is re-exposed from gateway.run, preserving the original object identity and logger namespace. The freshness predicate and its default remain in gateway.run because they depend on the timestamp coercion helper owned by another slice; leaving that dependency in place keeps this a narrow, byte-verbatim move rather than forcing an incomplete cluster transfer.

For refactor(gateway): extract interrupt helpers from run.py (slice 19 of #54962), GitHub reports check rollup SUCCESS on exact head b20be81408a66d9b7b23ad5d78fd39bc118cd43b.