Reconcile roadmap, compatibility matrix, and API inventory after latest coverage changes #255

Context

The default-branch documentation needed reconciliation after the API coverage and integration changes in issues #228–#238.

Audit findings and resolution in PR #257

• Replace stale active roadmap rows for already-closed issues with current open work (Add opt-in isolated SolrCloud verification for SPLITSHARD and MIGRATE #232, Triage remaining documented API gaps into roadmap priorities or explicit non-goals #256); keep the v4 release gate (Prepare v4.0 stable release checklist and changelog #110) separate.
• Correct the Collections compatibility summary: RELOAD is integration-tested on all three pinned versions; MODIFYCOLLECTION, SPLITSHARD, MIGRATE, ADDROLE, and REMOVEROLE remain unverified.
• Mark config.get and config.getOverlay as tested on Solr 8.11.4/9.11.0/10.1.0 and point to the exact integration tests that assert read-back and cleanup.
• Align ReplicationHandler docs and inventory with fetchindex / abortfetch across pinned versions. The one-shot leader/follower fetch and follower query read-back passed CI #640; live abort/polling remain unverified.
• Repair malformed literal newline separators in API coverage, compatibility, and Collections Markdown tables.
• npm run test:api-inventory and npm run docs:check passed in PR CI run #644 (30/30 jobs). The test (24) release:check step validated 31 API inventory entries, 139 local Markdown links, 68 JavaScript/TypeScript examples, and 5 package export targets.
• Confirm the documentation distinguishes the verified one-shot ReplicationHandler fetch from unverified live abort/polling, and keeps SPLITSHARD/MIGRATE unverified pending manual runtime evidence.

PR: #257. SPLITSHARD/MIGRATE remain explicitly unverified until the opt-in workflow has actually passed for each pinned Solr version.