feat(kv): add DiscardPolicy knob (default New, opt-in Old) — v0.7.2

[jaredLunde]

What

Adds a DiscardPolicy knob to StoreConfig so a bucket can choose what it does at max_bytes:

• New (default, unchanged) — reject writes when full (discard:new). Right for config buckets (certs/configs read as the source of truth) where silently dropping a live value is unacceptable. But it freezes writers at capacity (err 10077), so pair with max_age.
• Old (opt-in) — evict the oldest when full (discard:old): a hard size ceiling that never rejects. Right for high-churn log buckets whose consumers hold the durable fold (routing origins) — the bucket is a bounded change-feed, not the source of truth, so an evicted entry is recovered from the fold + CursorExpired resync.

Why

The routing-origins buckets (edge-origins/tunnel-origins) are high-churn logs: every deploy/evacuation writes a new route key and removes the old, so distinct keys (and their delete markers) grow monotonically. With the hardcoded discard:new and no trim, they grow to max_bytes and then reject all writes → traffic-routing writes freeze fleet-wide. max_age is the wrong lever (bucket size becomes a function of churn rate). The right fix is a size-bounded, evict-oldest log — which discard:old provides, and which is safe precisely because slipstream consumers keep the durable fold.

How

• StoreConfig.discard: DiscardPolicy, default New — every ..Default::default() caller is unchanged.
• async-nats' kv::Config has no discard field, so the normal create path can only produce discard:new. For Old, store_with_config skips it and uses the raw stream API (the only place the policy is expressible).
• Integration test (discard_policy_old_evicts_default_rejects) asserts a created Old bucket reads back discard:old and the default stays discard:new. All 45 integration tests pass against a real nats-server; clippy clean.

Versioning

0.7.1 -> 0.7.2. Additive (new field with Default, new enum, new export) — stays within consumers' ^0.7 requirement so they pick it up automatically.

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