[ci] Add import-time regression check for esphome.__main__

Adds a CI gate that runs `python -X importtime -c "import esphome.__main__"`
via importtime-waterfall's best-of-N harness, compares the root cumulative
time against a checked-in budget (script/import_time_budget.json, seeded
at 75.2ms with a 15% margin), and fails the build when top-level imports
regress.

The CLI pays this cost on every invocation before the requested command
even runs, so silently adding a top-level dep chain (the recent zeroconf
move in #13135 being the motivating case) hurts every user. The check
gives us a signal without waiting for user reports.

- script/check_import_time.py: --check / --update / --har PATH modes.
  On regression, prints a ranked top-15 offenders table by self-time.
- script/import_time_budget.json: baseline + margin_pct.
- script/determine-jobs.py: should_run_import_time() gates the job on
  esphome/**/*.py, requirements.txt, requirements_dev.txt, pyproject.toml,
  or changes to the check itself.
- .github/workflows/ci.yml: new import-time job, runs when gated and
  uploads a waterfall HAR artifact (14-day retention) for inspection.

script/import_time_budget.json

@@ -0,0 +1,5 @@
+{
+  "target_module": "esphome.__main__",
+  "margin_pct": 15,
+  "cumulative_us": 75199
+}