Improve fragment bundle

[xinz]

Improve fragment bundle:

1. Void repeated descendant metadata indexing in fragment bundling;
2. Improve fallback-anchor accumulation;
3. Skip identity-only external-reference rewrites.

Synthetic bundling benchmarks—not general validation speedups:

Workload	Before	After
Nested schema, depth 400	43.67 ms	0.49 ms
2,000 repeated anchor candidates	32.88 ms	0.87 ms
Identity-alias payload, size 2,000	12.95 ms	0.69 ms
Actual-redirect payload, size 2,000	15.95 ms	16.03 ms