fix(extension): render transitively referenced footnotes

[fzlzjerry]

Refs: #584

The v2 footnote renderer collected document references but skipped references inside footnote definitions. A nested reference could therefore point to a definition that was never rendered; physically nested definitions could also cause a panic.

This indexes definitions separately, then follows references reachable from the document. Each newly referenced definition is queued once, so cycles terminate and unused definitions stay hidden. Leaf definitions are not rescanned, and backlinks retain their existing reference-index order.

Parser numbering and the separate v1 ordering question in #410 are unchanged.

Validation

• Eleven regression/control cases cover chains, shared children, self-reference, cycles, nested definitions, prefixes, counts, and hidden unused notes. Nine fail on the original code; all pass with the fix.
• make test passes with Go 1.25.14, 1.26.8 and 1.27.1, including README examples.
• Full go test -race ./... passes.
• make lint passes with both root and _benchmark/go modules in a local workspace; no tracked module files change.
• 1,000 additional fixed-seed graph cases pass; the baseline fails 752.
• make bench ran before/after. Added BenchmarkFootnoteRender makes the changed extension path reproducible; the default CommonMark benchmark does not link this extension.

Ten balanced-order samples of the new 50-note benchmark show the following allocation costs:

Case	Before allocs/op	After allocs/op
flat	662	664
unused	555	507

The flat case pays a small queue/stack allocation cost. Timings vary across runs, so this is not a general speedup claim.

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