fix(storer): unstick radius decrement on live networks

[martinconic]

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Description

Nodes whose local storageRadius drifts above the neighborhood consensus stay stuck there indefinitely. Salud (pkg/salud/salud.go:243) compares CommittedDepth = capacityDoubling + Radius against the network mode with zero tolerance, flips the node unhealthy on any mismatch, and redistribution stops playing rounds. The reporter of #5428 observed this on bee 2.7.1-rc2 after an RPC instability event; the reporter of #5396 reproduced the same condition with code-level analysis.

The reserve worker's radius-decrement gate db.syncer.SyncRate() == 0 is structurally unreachable on a live mainnet. SyncRate() measures historical sync only, but historical sync never goes quiet for the 15-minute rate window because (1) peer churn keeps starting new cursor-based historical workers and (2) every successful decrement calls puller.onChange → resetIntervals(prevRadius) which discards non-neighbor sync history and forces re-historical-sync, bumping SyncRate above zero for the next ~15-30 min. The gate locks closed permanently. Nodes whose local Radius drifts above the neighborhood consensus stay there indefinitely — salud (zero-tolerance comparison of CommittedDepth = capacityDoubling + Radius) flips the node unhealthy and redistribution stops playing rounds. Restart doesn't help because waitNetworkRFunc re-loads the wrong local value.

Capacity-doubled nodes are structurally worse off: they pull more data, their SyncRate floor is higher, and the +1 from doubling amplifies any Radius mismatch into CommittedDepth mismatch.

Three changes, applied together:

1. Remove the SyncRate gate. The threshold check (count < threshold(capacity)) is already a correctness signal — if less than half of capacity sits within radius, the radius is wrong, regardless of sync activity. The unreserve path raises radius without consulting SyncRate; the asymmetry was unjustified.

2. Allow a two-step decrement when count is far below threshold (count*4 <= threshold). Under approximately uniform CAC bin distribution, each step doubles count-within-radius, so two steps land at or below threshold rather than overshooting. Capped at two to keep behavior bounded; each radius change triggers one resetIntervals so multi-step in one tick is strictly cheaper than the equivalent single-step ticks 5 minutes apart.

3. Reduce reserveWakeUpDuration from 15 to 5 minutes. With the gate gone, faster ticks bound worst-case recovery; tick cost is one countWithinRadius iteration.

The existing test "radius doesn't change due to non-zero pull rate encoded the buggy behavior as an invariant. Renamed and inverted to assert the new contract: with an empty reserve, the worker decrements toward minimumRadius regardless of the syncer's reported rate.

No protocol, storage layout, persistence format, or interface changes. The Syncer.SyncRate() interface stays — it's still used for "fully synced" determination at startup and for the status protocol's pullsyncRate field, both correct uses.

Fixes #5396
Fixes #5428

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