feat(ds): introduce BlobStoreFold

[thomasathorne]

Closes TZX-105.

What

Introduce the BlobStoreFold folder type, which merkelises any Foldable type into any store that implements BlobStore.

Why

For the new PVM Storage, we now have an on-disk implementation of BlobStore, so using this new fold we can merkelise an entire Pvm<Normal> onto disk---the "MVP" for a PVM snapshot that respects the merkle tree structure of the state.

Manually Testing

make all

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Explain changes to regression test captures when applicable.
• Write commit messages in agreement with our guidelines.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.

[github-actions]

Benchmark results for revision a6ff4bb:

Metric	Duration	TPS
Mean	1.515520125s	26.394
Worst	1.531360984s	26.121
Best	1.508570815s	26.515
Standard Deviation	±5.719671ms	±0.099

Full results

Run	Transfers	Duration	TPS
1	40	1.529181604s	26.158
2	40	1.51344225s	26.430
3	40	1.518758058s	26.337
4	40	1.512731127s	26.442
5	40	1.518761993s	26.337
6	40	1.513907587s	26.422
7	40	1.519368133s	26.327
8	40	1.508666304s	26.513
9	40	1.510840492s	26.475
10	40	1.51368192s	26.426
11	40	1.513800622s	26.424
12	40	1.512985144s	26.438
13	40	1.513575449s	26.427
14	40	1.531360984s	26.121
15	40	1.517538076s	26.358
16	40	1.515639007s	26.392
17	40	1.511605999s	26.462
18	40	1.512580285s	26.445
19	40	1.508570815s	26.515
20	40	1.513406653s	26.430

Compare the results above with those for the default branch.

[codecov]

Codecov Report

❌ Patch coverage is 94.79167% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.75%. Comparing base (ab5bf0d) to head (678f72b).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
data/src/store/fold.rs	94.79%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #965      +/-   ##
==========================================
- Coverage   89.78%   89.75%   -0.04%     
==========================================
  Files         110      111       +1     
  Lines       22104    22232     +128     
  Branches    22104    22232     +128     
==========================================
+ Hits        19846    19954     +108     
- Misses       1859     1880      +21     
+ Partials      399      398       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.