Add src/mongo/db/ifr_flag_registry.yml, an auto-generated registry that
tracks all Incremental Feature Rollout (IFR) feature flags in the `rollout`
or `released` incremental rollout phases. Gate the file behind a
CODEOWNERS rule that requires @10gen/rollout-flag-managers review
on every change, so phase transitions get visibility and an approval gate
without manual bookkeeping.

What changed

• Add buildscripts/idl/generate_ifr_registry.py. The script walks all
  .idl files under src/, collects feature flags declared with
  `incremental_rollout_phase: rollout` or `released`, and writes a
  deterministic, sorted YAML registry to
  src/mongo/db/ifr_flag_registry.yml.
• Call gen_ifr_registry from bazel/wrapper_hook/wrapper_hook.py before
  every build, regenerating the registry in place so it always
  reflects the current IDL state. Report status alongside the existing
  auto_headers output.
• Seed src/mongo/db/ifr_flag_registry.yml with the three current
  rollout flags (featureFlagCostBasedRanker, featureFlagMultiPlanLimiter,
  featureFlagUnifiedWriteExecutor) and an empty `released` section.
• Update CODEOWNERS and OWNERS.yml so any change to
  ifr_flag_registry.yml requires @10gen/rollout-flag-managers approval.

How to test

• Run `python3 buildscripts/idl/generate_ifr_registry.py` from the repo
  root and verify src/mongo/db/ifr_flag_registry.yml regenerates
  correctly.
• Run the unit tests: `python3 -m pytest buildscripts/idl/tests/test_generate_ifr_registry.py`.
• Trigger a normal bazel build and confirm the wrapper hook reports
  `[ifr_registry] (wrote)` or `[ifr_registry] (nochange)`.