refactor(fspy): introduce fspy_shm facade

[wan9chi]

Motivation

Isolate fspy shared-memory ownership behind a small internal facade so platform backends can be replaced incrementally without changing the channel layout or synchronization protocol.

Background

fspy_shared previously depended directly on shared_memory::Shmem and ShmemConf. That coupling made the Linux /dev/shm fix inseparable from channel code and forced every platform migration to touch the same low-level data path.

Changes

• Adds the private fspy_shm crate with create, open, identity, length, pointer, and slice APIs.
• Delegates to the existing shared_memory implementation on all platforms, preserving behavior in this PR.
• Migrates fspy_shared production code and real shared-memory tests to the facade.
• Adds create/open, cross-process visibility, owner cleanup, and opened-view lifetime coverage.
• Refreshes vulnerable lockfile entries for anyhow, crossbeam-epoch, and memmap2.

[wan9chi]

• perf(fspy): use sparse Windows shared memory #524: 2 dependent PRs (#525, #526)
• fix(fspy): use memfd for Linux shared memory #523
• test(e2e): reproduce constrained dev shm failure #522
• docs(fspy): document shared-memory facade ownership semantics #521
• refactor(fspy): introduce fspy_shm facade #520 👈 (View in Graphite)
• main

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[wan9chi]

Merge activity

• Jul 9, 2:10 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 9, 2:11 PM UTC: Graphite rebased this pull request as part of a merge.
• Jul 9, 2:15 PM UTC: @wan9chi merged this pull request with Graphite.