test: cover nextSubtreeSize

[loglapa]

Closes #265

Summary

• add direct table-driven coverage for nextSubtreeSize
• cover ranges limited by start alignment and by remaining length
• cover zero, unaligned, and larger aligned starts

Testing

• go test ./...
• go vet ./...
• golangci-lint run --timeout 10m
• git diff --check

[rach-id]

Use this prompt to include the relevant test cases:

Update PR #335 to strengthen TestNextSubtreeSize in proof_test.go.

The current table only uses power-of-two range lengths, and every positive even start is itself a power of two. Add table cases that exercise:

• Rounding a non-power-of-two remaining span down, such as (start: 0, end: 12) → 8 and (start: 8, end: 14) → 4.
• Alignment for a composite even start, such as (start: 12, end: 20) → 4.

Keep the change test-only unless investigation reveals an actual implementation defect. Use descriptive case names consistent with the existing table.

After editing, run:

go test ./...
go vet ./...
git diff --check

Review the final diff to ensure the new cases independently constrain both behaviors implemented by nextSubtreeSize: start alignment and rounding based on the remaining range.

[loglapa]

Addressed the requested coverage in a0841be: added independent cases for non-power-of-two remaining spans at (0, 12) and (8, 14), plus composite-even start alignment at (12, 20). Validation passed: go test ./..., go vet ./..., and git diff --check.