perf(fspy): use sparse Windows shared memory

[wan9chi]

Motivation

Avoid allocating a fully backed 4 GiB temporary file for every tracked task on Windows.

Background

The parity backend deliberately retains the previous file-backed resource model. Marking that file sparse allows the logical mapping to remain 4 GiB while physical allocation follows the small prefix actually used by fspy records.

Changes

• Creates the owner backing file as temporary and delete-on-close.
• Calls FSCTL_SET_SPARSE before extending the file.
• Falls back to ordinary sizing for filesystems that explicitly report sparse files as unsupported.
• Continues to propagate access-denied, invalid-parameter, and other unexpected sparse setup errors.
• Opens sender views directly through the decoded named mapping without reopening or recomputing the backing path.
• Orders owner fields so the view, mapping handle, and backing file close safely.
• Adds logical-versus-allocated-size, endpoint allocation, sparse fallback, hard-error, direct-open, changed-environment, and cleanup coverage.
• Adds the Windows performance changelog entry.

[wan9chi]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• perf(fspy): use sparse Windows shared memory #525 👈 (View in Graphite)
• perf(fspy): use sparse Windows shared memory #524: 1 other dependent PR (#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
• main

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