/claim #228

Please be sure to look over the pull request guidelines here: https://github.com/spaceandtimelabs/sxt-proof-of-sql/blob/main/CONTRIBUTING.md#submit-pr.

Please go through the following checklist

• The PR title and commit messages adhere to guidelines here: https://github.com/spaceandtimelabs/sxt-proof-of-sql/blob/main/CONTRIBUTING.md. In particular ! is used if and only if at least one breaking change has been introduced.
• I have run the ci check script with source scripts/run_ci_checks.sh.
• I have run the clean commit check script with source scripts/check_commits.sh, and the commit history is certified to follow clean commit guidelines as described here: https://github.com/spaceandtimelabs/sxt-proof-of-sql/blob/main/COMMIT_GUIDELINES.md
• The latest changes from main have been incorporated to this PR by simple rebase if possible, if not, then conflicts are resolved appropriately.

Rationale for this change

This PR addresses checklist item 2 from #228 only.

The issue asks to remove the generic string conversion bounds from Scalar and replace them with an explicit from_str_via_hash(&str) path. This keeps string hashing behavior explicit at the trait level instead of relying on blanket From<&str> / From<String> bounds.

What changes are included in this PR?

• Add Scalar::from_str_via_hash(&str) with a default implementation backed by the existing byte-slice hash helper.
• Remove the generic string From bounds from Scalar.
• Route generic VarChar string-to-scalar conversions through from_str_via_hash in database, commitment, proof, and Arrow conversion paths.
• Add a regression test that checks from_str_via_hash("abc") matches the existing byte-hash path.

Are these changes tested?

Yes.

I ran the following locally:

• cargo fmt --check
• cargo check -p proof-of-sql --no-default-features
• cargo check -p proof-of-sql --no-default-features --features="std"
• cargo check -p proof-of-sql --no-default-features --features="test"
• cargo check -p proof-of-sql --no-default-features --features="arrow"
• cargo check -p proof-of-sql --no-default-features --features="rayon"
• cargo check -p proof-of-sql --all-targets --no-default-features --features="test"
• cargo check -p proof-of-sql --all-targets --no-default-features --features="arrow"
• cargo check -p proof-of-sql --all-targets --no-default-features --features="rayon"
• cargo test -p proof-of-sql we_can_get_scalar_from_hashed_strings --no-default-features --features="test"
• cargo test -p proof-of-sql mixed_data_types --no-default-features --features="arrow test"

Note: source scripts/run_ci_checks.sh currently does not execute cleanly in this checkout because it extracts literal run: prefixes from the workflow YAML. I ran it, then ran the relevant cargo checks above directly. Default/perf feature lanes are also not fully runnable on this host because the dependency stack includes Linux-only / x86_64-only paths (blitzar-sys, halo2curves asm).