perf: make event-head upgrades lazy

[steipete]

Summary

• replace the eager message-event head backfill with an atomic lazy trigger
• use a compact WITHOUT ROWID head table and skip unchanged head rewrites
• centralize message-row projection, scanning, and mention hydration across read paths
• add an explicit schema v5 migration for databases created by the released v4 schema

Verification

• GOWORK=off go test ./...
• large-archive v3-to-v5 open completed in under 100 ms with zero eager head rows
• structured Codex autoreview: clean, no accepted/actionable findings

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: perf: make event-head upgrades lazy This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.