Add Datadog 2026 interview practice problems

[perixtar]

Summary

• add FastPrep practice links for Delete a Filesystem Tree, Query Log Match, and Latency Bucket Counter
• place all three entries by their latest 2026 reported dates
• refresh time-sensitive README freshness badges

Validation

• python3 scripts/refresh-freshness.py (second pass: 0 marker changes, 0 reordered rows; README 496,844 bytes)
• /opt/homebrew/bin/python3.12 scripts/test_sync_practice_formats.py (11 tests passed)
• README contract check: 2,133 rows, 2,133 unique routes, descending dates, one sorted Datadog company entry, exact two-link/one-row shape for each new route, and size below 500,000 bytes
• all three FastPrep routes returned HTTP 200 and their APIs match the intended title, company, and date
• git diff --check

Independent review

PASS with no P0–P3 findings on exact head 3669adbcb06aa7f404f956c9f4940d5e545802ac against exact base 1450b8f42f9ce3fce885a7696b72e33334a3ad33. The reviewer checked the complete README-only diff, row uniqueness and shape, descending date order, company-list integrity, freshness idempotence, repository unit tests, file size, and live route/API metadata.

Known generated-mirror drift

/opt/homebrew/bin/python3.12 scripts/sync-practice-formats.py --check is already nonzero on pristine origin/master: the README formats projection plus formats/system-design.md and formats/low-level-design.md are stale. On this README-only branch, formats/coding.md is additionally stale because of the three new coding rows. Generated format pages are intentionally outside this focused README sync.