perf(sync): coordinate startup across processes

[pablontiv]

What

• replace quadratic startup tagging concatenation with an incremental accumulator
• coordinate startup sync across processes with a persistent 0600 advisory lock
• allow compatible snapshot/config followers to continue immediately while mutations wait up to five seconds
• make snapshot query preparation read-only and non-migrating
• add unit, subprocess, crash-recovery, race, and Windows cross-compile coverage
• document the owner/follower contract and package inventory

Why

Concurrent Backscroll invocations independently repeated expensive startup work and could contend on SQLite. Large sessions also triggered O(n²) string concatenation during tagging, causing commands to exceed 120 seconds and consume excessive transient memory.

Closes #46.

How

• added tagging.Accumulator to classify each message without constructing a session-sized string
• added internal/startuplock using github.com/gofrs/flock v0.13.0, keyed by the canonical database path
• classified all operational commands as snapshot reads, metadata reads, or mutations
• centralized owner/follower coordination in the root startup policy
• retained mutation leases through handlers, including recover post-install sync
• routed busy snapshot followers through compatible OpenReadOnly WAL inspection and emitted freshness warnings only on stderr
• added deterministic process barriers proving single-flight ownership, timeout behavior, and crash recovery

Design: docs/superpowers/specs/2026-08-21-startup-sync-coordination-design.md
ADR: docs/adr/0003-coordinar-sync-entre-procesos.md

Verification

• rootline validate docs/adr/0003-coordinar-sync-entre-procesos.md --strict
• just check
• go test -race ./...
• just test
• just ci — aggregate coverage 86.5%
• repeated subprocess/process-death tests (-count=5)
• GOOS=windows GOARCH=amd64 go test -c ./internal/startuplock
• accumulator benchmark scales linearly from 128 to 512 records at ~2.28 MB/s

Checklist

• Tests pass (just test)
• Code is clean (just check)
• Snapshots updated if needed (N/A — no snapshot suite)
• Changes are documented if user-facing