v2.8.0 — Hyperion operations: template persistence, real ES watermarks, act_digest mapping, restart gaps

Hyperion operations corrections from running the XPR mainnet full-history node in production. MINOR bump: one correction to existing advice plus new reference content. Every claim was checked against the Hyperion 4.0.8 source on the node and against live cluster settings.

Fixed — hyperion-operations-caveats.md §0/§5

• "Edit the legacy template with PUT _template/proton-action" does not persist: the indexer master re-PUTs every legacy template from index-templates.ts on each start (updateIndexTemplates()). The advice now says to edit the definitions file (src/ and build/), restart gracefully and verify, and to treat it as a local patch that must be re-applied after upgrades.

Added

• §3: the real ES disk watermarks (low/high/flood) and their max_headroom caps, which make the high watermark trip at 150 GB free on large drives (earlier than "90%"). Includes the absolute-watermark settings and advice to alert on free space at the next 10M-block partition boundary. hyperion-setup.md operations note corrected to match.
• §5: _disk_usage to find what actually uses the bytes; act_digest is dynamically mapped as text + keyword in 4.0.8 (~26% of an action partition). Mapping it as keyword without doc_values keeps it in _source, in responses and searchable, and cuts ~18% of future partition size. Also a note to keep data (e.g. eosio::onblock) rather than blacklist on public nodes.
• §11: every indexer restart leaves a block gap (the live reader resumes at head; a 30 s graceful restart left 59 missing blocks) — repair after restarts or automate quick-scan + fill-missing on a cron.
• §11: fill-missing --host takes the bare host; the headless clearLine crash happens after the request is sent; alert on /v2/health missing_blocks > 0 (43 missing blocks appeared on a healthy live node); scan-actions takes ~15 s for full history.