[vsimko]

Summary

Reconcile the default-branch documentation after PR #253 merged.

• Replace stale roadmap priorities with the remaining open work (Add opt-in isolated SolrCloud verification for SPLITSHARD and MIGRATE #232, Reconcile roadmap, compatibility matrix, and API inventory after latest coverage changes #255, Triage remaining documented API gaps into roadmap priorities or explicit non-goals #256) and keep the v4 release gate (Prepare v4.0 stable release checklist and changelog #110) separate.
• Update compatibility and API coverage to record the one-shot ReplicationHandler fetch/read-back as tested on Solr 8.11.4, 9.11.0, and 10.1.0 (CI #640), while retaining explicit non-certification for live abort/polling.
• Correct Config GET and Config overlay inventory statuses and point them at the integration tests that assert read-back and cleanup.
• Align ReplicationHandler guide and endpoint inventory with the canonical fetchindex / abortfetch command usage and follower BasicAuth custom parameters.
• Repair malformed literal newline separators in compatibility/API/Collections Markdown tables.

Validation / boundaries

• The evidence references the merged PR test(solr): verify replication and shard lifecycle operations #253 and its green 30/30 CI run #640.
• PR validation run #644 passed 30/30 jobs; npm run test:api-inventory and npm run docs:check passed, validating 31 inventory entries, 139 local Markdown links, 68 JS/TS examples, and 5 package export targets.
• SPLITSHARD and MIGRATE remain runtime-unverified until the opt-in workflow_dispatch has actually run successfully per pinned Solr version.
• No release or package publication is part of this PR.

Closes #255.