fix: serialize unsafe backend structural mutations

[pathscale]

Summary

• gate Arctic and Congee structural mutations with per-index read/write exclusion
• keep point and range reads concurrent while preventing reads from observing structural rewrites
• add shared-instance and separate-instance backend concurrency regressions

Evidence

Before the fix, repeated whole-library runs reproduced:

• Arctic raw-cursor structural assertion (repetition 59)
• missing immediate Congee read (repetition 17)

After the fix:

• focused unique-index tests pass
• whole-library stress passes 100/100
• aggregate four-backend stress passes 500/500
• isolated WorkTablesIndex and upstream IndexSet stress each pass 500/500
• full workspace tests pass: 189 library, 522 integration with 4 ignored, 67 codegen, and doctests
• cargo fmt and warnings-denied Clippy pass

Caveat

A longer full-library campaign observed one rare WorkTablesIndex immediate-read miss at repetition 249. This PR does not change that backend; the added diagnostics preserve the evidence for follow-up review.