feat(codec): update fixed binding indexes with immutable CoW

[Ivanbeethoven]

The canonical binding-index codec can describe a root but has no immutable mutation or lookup engine. Add iterative update, exact get and component-aware longest_prefix operations using the fixed v1 node/key encoding. Updates copy the changed path's ancestors; deletion compresses valueless paths, identical updates write no nodes, insertion order and delete/reinsert produce identical canonical roots, and old roots retain their previous mapping.

Stacked on #9. The synchronous caller-owned NodeStore port reads immutable bytes and inserts immutable nodes; there is no concrete persistence store, SQL/publication transaction, async runtime adapter, pagination, namespace serving or capability enablement. Each visited node checks its requested SHA-256 identity, local shape, incoming child label and assembled value-key budget. Unvisited subgraphs, source membership, complete closure, authorization and retention remain separate integration gates. NodeStore::read returns caller-allocated bytes: the codec's post-read size check is not a bounded storage allocation guarantee. A failed update returns no new root; partial insertions may leave unreachable nodes for the caller's transaction/retention policy.

Validation on Windows, Rust 1.98.1:

• Formatting and git diff --check: pass.
• Identity profile: all-target clippy with warnings denied, build and complete tests pass (81 unit + 6 integration).
• Zstd profile: all-target clippy with warnings denied, build and complete tests pass (101 unit + 6 integration).
• Eleven new tests use an independent ordered-map/component-vector oracle for 100 fixed mutations and historical roots; insertion-order/delete-reinsert identity, untouched sibling reuse, no-op writes, missing/corrupt/oversized nodes, invalid incoming edges/value paths, UTF-8 split labels, exact/deep key limits, failed insert/retry and missing sibling during collapse are covered. Store call counts are logical test-store observations, not SQL, runtime latency or crash evidence.
• At exact head 9bb7c1ac5fbfb7dec94d4e79cd9a9e365168cf47, all six Linux/macOS/Windows × identity/zstd jobs passed: https://github.com/gitmono-dev/mst2-codec/actions/runs/37258540385 . Two independent static reviews found no blocking CoW/index correctness issue.

Only Rust source/tests are pushed. The private plan and evidence stay local. This does not complete T02/T04/T05 or claim an end-to-end performance gain.