feat: adopt PostgreSQL's naming convention of temporary files and directories

[usamoi]

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[Copilot]

Pull request overview

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Adopts PostgreSQL-like naming/location for temporary files/directories used during BM25 index maintenance/build operations.

Changes:

• Added a Postgres-style temp name generator and new temp file/dir utilities under src/index/temp.rs.
• Updated BM25 maintenance/vacuum and build paths to use the new temp dir/file plumbing.
• Adjusted workspace dependency usage (notably getrandom/rand) and removed tempfile from core workspace deps.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/lib.rs	Adds tempname() generator used for PG-style temp paths.
src/index/temp.rs	New temp file/dir helpers that create artifacts under temp tablespaces.
src/index/mod.rs	Registers the new temp module.
src/index/bm25/am/am_vacuumcleanup.rs	Passes temp dir/file paths into BM25 maintenance.
src/index/bm25/am/am_build.rs	Switches build temp directory creation to the new temp helper; reseeds RNG in parallel worker entry.
crates/bm25/src/maintain.rs	Changes maintain() signature to accept temp dir/file paths and uses them for IO.
crates/xtask/Cargo.toml	Pins tempfile directly instead of workspace dependency.
crates/bm25/Cargo.toml	Switches getrandom to workspace; removes tempfile workspace dep usage.
Cargo.toml	Adds getrandom/rand workspace usage and removes tempfile from workspace deps.

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