Backfill architectural ADRs

[osvaldoandrade]

Document key design decisions retroactively.

Goal: Record rationale for existing architecture choices.

ADRs to write:

• 0006-reconciliation-throttle.md - Why throttle delayed→pending sweep per (cmd,tenant)
• 0007-bloom-idempotency.md - Bloom filter for fast idempotency checks
• 0008-lease-table-asymmetry.md - In-memory (Pebble) vs Redis lease tracking
• 0009-mux-transport.md - Sharing one TCP port for multi-shard Raft
• 0010-per-shard-fsm.md - Separate FSM per Raft group

Acceptance criteria:

• ADRs follow template
• Context, decision, consequences, alternatives documented
• Merged into docs/adr/

Part of: #655