chore(basics/counter): remove legacy mpl-stack (Shank+Solita) example

[mikemaccana]

Summary

Removes basics/counter/mpl-stack — a native counter that demonstrated the Metaplex "stack" (Shank IDL → Solita TS client). It's redundant and was the one example no CI workflow covered:

• Its dir name (mpl-stack) isn't in any workflow's discovery set (native/anchor/quasar/pinocchio/asm), so it was never functionally tested — only host-linted as a workspace member. Its test is jest against a local validator, which can't run in CI in-process anyway.
• Its counter logic is already shown by basics/counter/{native,anchor,pinocchio,asm} (all CI-tested).
• Its Shank → generated-client lesson is now covered with the current tool by tools/shank-and-codama — Solita is unmaintained (last release Nov 2023; superseded by Kinobi → Codama).

So it adds no unique teaching value and was a permanent CI blind spot. Removing it (and its root Cargo.toml workspace-member entry) closes the last gap found in a full repo sweep — .github/.ghaignore is empty, and every remaining example dir is discovered by a framework workflow.

cargo metadata confirms the workspace still resolves after removal.

https://claude.ai/code/session_013dpnF6uSGWXjkJJZseqzcP

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