chore(grep): prepare 1.21.2 release

[AlexMikhalev]

Summary

Prepare the corrected terraphim_grep 1.21.2 publication after truthful insufficient-result handling merged in #9.

• pin only terraphim_grep to 1.21.2;
• document preserved chunks/KG concepts and truthful counters;
• leave other workspace package versions unchanged.

Verification

• cargo +stable fmt --all -- --check
• cargo +stable test -p terraphim_grep — 61 tests passed, 0 failed; 1 doctest ignored
• cargo +stable clippy -p terraphim_grep --all-targets -- -D warnings
• git diff --check

The raw local cargo publish --dry-run correctly rejects the private terraphim_service registry reference. Publication must use the existing Publish Crates workflow, whose dedicated pre-publish step strips private-registry refs before invoking Cargo.

Release gate

After merge, dispatch .github/workflows/publish-crates.yml with:

• crate_list=terraphim_grep
• dry_run=false

Then install the exact crates.io 1.21.2 artifact into a clean root and rerun the structured-output smoke.