[nextest-runner] store records in the state dir, not the cache dir

[sunshowers]

The state dir is a more reasonable place to put records, since they're not regenerable.

Also add a migration path. This won't affect too many folks but I want to put it in place while the impact is minimal. I'll remove the migration soon.

[Copilot]

Pull request overview

This PR migrates nextest's test run recordings from the cache directory to the state directory, which is more semantically appropriate since recordings represent accumulated state that cannot be regenerated, not cache data.

Changes:

• Renamed records_cache_dir to records_state_dir and moved from cache_dir.rs to state_dir.rs
• Renamed CacheDirError to StateDirError and updated all error messages
• Added automatic one-time migration from ~/.cache/nextest/ to ~/.local/state/nextest/ on Unix systems
• Updated all references across the codebase including internal tools and integration tests

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
site/src/docs/features/record-replay-rerun.md	Updated documentation to reflect new state directory location
nextest-runner/src/record/state_dir.rs	Renamed from cache_dir.rs; added migration logic and comprehensive tests
nextest-runner/src/record/session.rs	Updated to use records_state_dir and state_dir terminology
nextest-runner/src/record/mod.rs	Updated module declarations and exports
nextest-runner/src/errors.rs	Renamed CacheDirError to StateDirError and updated error messages
internal-tools/zstd-dict/src/main.rs	Updated internal tool to use state directory logic
integration-tests/tests/integration/record_replay.rs	Updated test comments and documentation
cargo-nextest/src/errors.rs	Updated error type references
cargo-nextest/src/dispatch/utility/store.rs	Updated to use records_state_dir
cargo-nextest/src/dispatch/core/run.rs	Updated to use records_state_dir
cargo-nextest/src/dispatch/core/replay.rs	Updated to use records_state_dir

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[codecov]

Codecov Report

❌ Patch coverage is 91.71975% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.01%. Comparing base (437f0f4) to head (3259b36).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/record/state_dir.rs	93.02%	9 Missing ⚠️
cargo-nextest/src/errors.rs	0.00%	2 Missing ⚠️
nextest-runner/src/record/session.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3043      +/-   ##
==========================================
+ Coverage   83.98%   84.01%   +0.02%     
==========================================
  Files         154      154              
  Lines       41768    41873     +105     
==========================================
+ Hits        35079    35178      +99     
- Misses       6689     6695       +6     

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