feat(mkLogosModule): auto-stage external_libraries in dev shell

[danisharora099]

Closes #97.

What

When the consumer passes a non-empty externalLibInputs, mkLogosModule's dev shell now:

1. Builds the external libraries (reusing the same resolveExtInput + mkExternalLib.buildExternalLibs path the package outputs already use).
2. Symlinks each one's lib/* and include/*.h into ./lib so the module's existing CMakeLists.txt (find_library(... PATHS lib NO_DEFAULT_PATH)) resolves it outside the Nix sandbox.
3. Exports DYLD_LIBRARY_PATH (Darwin) / LD_LIBRARY_PATH (Linux) + CMAKE_LIBRARY_PATH + CMAKE_INCLUDE_PATH.
4. Exports CMAKE_EXPORT_COMPILE_COMMANDS=ON for clangd.

Why

Every universal C++ module wrapping a Rust cdylib (or any external lib declared in metadata.json#nix.external_libraries) currently writes the same dev-shell override to make nix develop work with clangd / IDEs / plain cmake. The most recent example is eth-lez-atomic-swaps PR #26. After this lands, that ~40-line block disappears downstream.

Full rationale + design questions in #97.

Backward compatibility

• Modules without external_libraries: zero change (the new behaviour is gated on the list being non-empty).
• Modules that already override devShells.<system>.default: keep working.
• nix build derivations: untouched. Only nix develop is affected.

Verification

• nix flake check passes on aarch64-darwin (all 4 checks, including devShells.aarch64-darwin.default).
• One file changed, +62 / −2.

Notes for review

A few small calls in here I'd like a second opinion on (also in #97):

• Symlink vs copy when staging into ./lib. Using symlinks here so a cdylib rebuild is picked up on the next shell entry; the in-sandbox copyExternalLibsToLib uses cp. Worth surfacing.
• Header glob is *.h only — happy to extend to *.hpp / extension-less outputs if anyone needs it.
• "Auto whenever externalLibInputs is non-empty" vs an explicit autoStageExternalLibs toggle. I went with implicit.
• resolveExtInput is duplicated locally inside devShells rather than hoisted, to keep the blast radius small. Happy to lift it as a follow-up if reviewers prefer.