generate: always use Pool

[pjonsson]

The imap implementation will be
as lazy as one could expect in
Python 3.15, but until then
it is more eager than one would
expect.

Switch from imap_unordered to
using apply_async instead, but
print the results in a deterministic
order and remove the special
case where a different code path
is taken when running on a single
core.

---

📚 Documentation preview 📚: https://datacube-explorer--862.org.readthedocs.build/en/862/

[codecov]

Codecov Report

❌ Patch coverage is 15.38462% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.24%. Comparing base (367aecf) to head (1c4c68d).

Files with missing lines	Patch %	Lines
cubedash/generate.py	15.38%	11 Missing ⚠️

❌ Your project check has failed because the head coverage (75.24%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (367aecf) and HEAD (1c4c68d). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (367aecf)	HEAD (1c4c68d)
	4	2

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #862      +/-   ##
===========================================
- Coverage    84.33%   75.24%   -9.10%     
===========================================
  Files           35       35              
  Lines         4214     4209       -5     
  Branches       535      532       -3     
===========================================
- Hits          3554     3167     -387     
- Misses         465      851     +386     
+ Partials       195      191       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[pjonsson]

This is failing because the Click CLI runner does not flush stderr as it should until Click 8.2.1, so the test output is not appearing as expected.

Unfortunately Explorer cannot use Click 8.2.1 because that changed something else causing multiple test failures, but I believe Click 8.3.0 will fix that issue.

Putting this as a draft for now until Click 8.3.0 is released.