fix(solana): optimize signature verification for Cosmoshub

[vaporif]

Description

• Swap find_program_address (brute-forces up to 256 bumps) for caller-supplied bumps + create_program_address (single hash) on chunk and signature verification accounts
• Match signature accounts by embedded sig_hash instead of PDA re-derivation — filtered by owner and discriminator first

Testing

• Layout sentinel test validates is_valid and sig_hash offsets survive field changes
• Unit tests cover wrong discriminator, wrong owner, wrong sig_hash all fall back correctly

e2e test via

export EXTERNAL_COSMOS_RPC_URL=https://cosmos-rpc.polkachu.com
export EXTERNAL_COSMOS_CHAIN_ID="cosmoshub-4"
just test-e2e TestExternalCosmos/Test_ExternalCosmos_UpdateClient

(might worth adding this to CI since this test never runs and was broken before)

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Wrote unit and integration tests.
• Added relevant natspec and godoc comments.
• Provide a conventional commit message to follow the repository standards.
• Re-reviewed Files changed in the GitHub PR explorer.
• Review SonarCloud Report in the comment section below once CI passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (d0ab571) to head (ee8d1db).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1022   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          27       27           
  Lines        1123     1123           
=======================================
  Hits         1122     1122           
  Misses          1        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.

[greptile-apps]

PR author is not in the allowed authors list.

[srdtrk]

add docs as to what this does. And maybe rename

[vaporif]

done

[srdtrk]

Lgtm. I'll leave it open for now due to the audit,