fix(p2p/server): reject range requests below the store tail

[renaynay]

Resolves PROTOCO-2383.

Rejects a GetRangeRequest whose from is below the store's tail with ErrNotFound up front, instead of loading headers until it runs into a pruned height.

WHY? A partial range must start at from (see interface.go partial-range contract), so a below-tail from can only be a NOT_FOUND.

Complements #405: the tail check guards the request bottom, #405's min(to, head+1) guards the top.

• One atomic Tail() load (atomic.Pointer), no datastore hit.
• Adds regression test TestExchangeServer_rangeBelowTail.

🤖 Generated with Claude Code

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[loglapa]

The merged change currently makes the existing TestExchangeServer_partialRangeNotExpanded panic deterministically on main before either normal or race suites can complete. Reproduced on Go 1.26/darwin and Go 1.27/linux-arm64:

panic: nil pointer dereference
p2p.(*partialRangeStore).Tail
p2p/server.go:237
p2p/server_test.go:198

handleRangeRequest now always calls Tail, while partialRangeStore embeds a nil header.Store and does not implement Tail. The existing test needs a concrete tail at or below from plus a Tail method (so it continues exercising the partial-range path rather than the new below-tail rejection). There is no open follow-up PR/issue yet.

[loglapa]

I opened a minimal follow-up fix in #409. It adds an explicit tail to the existing test double, preserves the original partial-range path, and restores the full suite on Go 1.27.