chore(deps): align rusqlite 0.40 + git2 0.21 with OpenHuman host

[senamakel]

Summary

Align the crate's native-linking dependencies with the OpenHuman host so the two can be built into one binary. This is the first required PR for the OpenHuman → TinyCortex memory migration (host tracking PR: tinyhumansai/openhuman#4516).

• Bump rusqlite 0.32 → 0.40 and git2 0.19 → 0.21 (with vendored-libgit2), matching the host pins (rusqlite = "=0.40.0", git2 = "0.21" + vendored-libgit2).
• Adapt the API deltas the bumps introduce.

Problem

rusqlite (→ libsqlite3-sys, links = "sqlite3") and git2 (→ libgit2-sys, links = "git2") are native-linking crates. Cargo hard-errors if two versions of the same links crate appear in one dependency graph. OpenHuman pins rusqlite =0.40.0 (bundled) and git2 0.21 (vendored-libgit2); TinyCortex pinned rusqlite 0.32 (bundled) / git2 0.19. With both bundling SQLite, the host could not even activate its tinycortex dependency — cargo refused to resolve the graph.

Solution

• rusqlite 0.40: >= 0.33 dropped FromSql for usize, so VectorStore::count reads COUNT(*) as i64 and converts via usize::try_from(...).context(...) (matches OpenHuman's own fix for the same call).
• git2 0.21 API deltas in diff/ledger.rs:
  • Tag::message() → Result<Option<&str>, _> — checkpoint tag reader degrades a missing/non-UTF8 message to "".
  • StringArray::Iter::Item → Result<Option<&str>, _> — the two tag-name loops use filter_map(|r| r.ok().flatten()).
  • Buf::as_str() → Result<&str, Utf8Error> — patch_text uses .ok()?.

Validation

• cargo check --all-targets — clean.
• cargo test --lib diff:: — 38 passed (exercises the changed checkpoint/tag-delete/patch code).
• Host root world (tinyhumansai/openhuman) cargo check with tinycortex = "0.1" active — exit 0, no multiple packages link to native library error (one bundled SQLite + one libgit2 confirmed).

Notes

Keep these pins in lockstep with the host root Cargo.toml going forward — a comment in Cargo.toml records why. No behavior change beyond the dependency bump + API adaptation.

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📥 Commits

Reviewing files that changed from the base of the PR and between d1a8c7b and 35f9518.

📒 Files selected for processing (3)

• Cargo.toml
• src/memory/diff/ledger.rs
• src/memory/store/vectors/store.rs

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