fix(spv): allow proofs longer than 144 headers

[mswilkison]

Summary

• remove the client-only 144-header cap from the SPV proof walk
• keep walking through mined headers until the proof has enough work or reaches the Bitcoin chain tip
• skip only headers whose decoded target exactly matches the Bridge DIFF1 target
• reject non-positive header targets before calculating difficulty

Regression coverage

• 144 DIFF1 headers followed by a decisive difficulty-16 header
• non-DIFF1 bits 0x1d00aaaa, whose integer difficulty rounds to one, remain decisive
• zero-target compact bits return a proof error instead of panicking

Testing

• go test -count=1 ./pkg/maintainer/spv
• go test ./pkg/maintainer/...

Stacked on #4039. The temporary base branch mirrors #4039 at 832c116; retarget this PR to main after #4039 merges.

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