component: avoid pre-allocating untrusted lengths in the binary set readers

[AlexandrKhromov2005]

component: avoid pre-allocating untrusted lengths in the binary set readers

ReadDomainSetBin (component/trie) and ReadIpCidrSet (component/cidr) read a length with
binary.Read and immediately allocate a slice of that size
(make([]uint64, length) / make([]byte, length) / make([]netipx.IPRange, length)) before
reading the elements. The length is only checked for < 1, not for an upper bound, so a crafted
binary set — reachable from an .mrs rule-set, including remote rule-providers
(rules/provider fetches provider content over HTTP and calls rulesMrsParse) — can declare a
huge length and either OOM the process on the allocation or panic with
makeslice: len out of range.

Found by fuzzing (go test -fuzz); the readers crash on a tiny input immediately.

Fix

Read incrementally instead of trusting the length for a single allocation:

• element slices grow via append with a capped initial capacity, so a bogus length hits EOF
  while reading the elements;
• the byte slice is read through io.CopyN into a growable buffer.

Adds FuzzReadDomainSetBin / FuzzReadIpCidrSet. Existing tests pass; valid sets still decode.

[AlexandrKhromov2005]

While re-auditing the .mrs parse path I found one more instance of the same pattern in the same code path, so I've added it to this PR (commit c51223f).

rules/provider/mrs_reader.go reads a reserved extra field as:

extra := make([]byte, length) // length is an untrusted int64, only checked for < 0
_, err = io.ReadFull(reader, extra)

length comes straight from the (attacker-controlled) rule-set, so a crafted .mrs can declare a huge length and abort the process (fatal error: runtime: out of memory, or a makeslice: len out of range panic) before any body is read — reachable earlier than the two set readers already fixed here. Since the extra bytes are currently unused ("reserved for future using"), the fix just discards them with io.CopyN(io.Discard, reader, length), which streams and returns a short-read error for a bogus length.

Added a regression test (mrs_reader_oom_test.go); go test ./rules/provider/ ./component/trie/ ./component/cidr/ passes.