FORM: make the read path thread-safe for multi-product form_source #1008

Summary

One form_source creates a provider per configured product. All providers share the same form_reader → persistence_reader → storage_reader. Each provider is concurrency::serial, which only serializes a provider against itself. Providers for different products can call the same storage_reader concurrently.

ROOT container reads are already guarded by root_tbranch_read_mutex and root_rfield_read_mutex. Those locks do not protect storage_reader's own state: files_, read_containers_, and index_maps_ can be looked up and inserted from several threads at once. This causes undefined behavior and can corrupt a map or crash the read.

This race predates #1007 and is independent of the cell-index lookup fix in #992. The old get_index() mutated index_maps_ through operator[] on every call. The new implementation reduces mutation to initial index-map construction but does not make the read path thread-safe.

Required changes

• Audit shared mutable state across form_reader, persistence_reader, and storage_reader.
• Protect lookup, initialization, and mutation of shared read-side state with a consistent synchronization strategy.
• Cover all externally callable storage-reader operations that access shared state, including get_index, list_indices, prime, and read_container.
• Ensure internal calls do not lock the same non-recursive mutex twice.
• Preserve the existing ROOT read locks and check lock ordering to avoid deadlocks.

Affected areas

• form/form_source.cpp: provider construction and shared-reader ownership.
• form/persistence/persistence_reader.cpp and form/persistence/persistence_reader.hpp: shared persistence-reader state.
• form/storage/storage_reader.cpp and form/storage/storage_reader.hpp: shared file, container, and index caches.
• test/form: multi-product parallel read-back regression coverage.

Acceptance criteria

• A regression test reads multiple products through one form_source with -j greater than 1.
• The test checks each product's value against its expected cell and exercises initial cache construction as well as subsequent reads.
• Shared read-side state remains protected when different product providers execute concurrently.
• Existing cell-index lookup and read-back behavior remains correct.
• Run the regression under a race detector where supported, and record the results.

References

• Read-side counterpart of Make FORM's write path thread-safe independent of Phlex #840 (write path).
• Related: Enable ROOT thread safety during FORM initialization #989 (ROOT::EnableThreadSafety).
• Cell-index lookup issue: FORM: form_source serves another data cell's products for single-layer files written out of order #992.
• Originating PR: FORM: locate every cell through the index container #1007
• Review comment: FORM: locate every cell through the index container #1007 (comment)

Requested by @wwuoneway.