fix: revert Rust toolchain version from 1.87.0 to 1.82.0

Fixes CI failure where workflows were trying to download Rust 1.87.0,
which doesn't exist yet. Reverts to 1.82.0, the last known working MSRV.

The erroneous 1.87.0 version was introduced in commit e3f082b during
the ndarray 0.16 upgrade.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: rathideep22

.github/workflows/checking.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         toolchain:
-          - 1.87.0
+          - 1.82.0
           - stable
           - nightly
         os:

.github/workflows/testing.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         toolchain:
-          - 1.87.0
+          - 1.82.0
           - stable
         os:
           - ubuntu-latest
@@ -37,7 +37,7 @@ jobs:
       fail-fast: false
       matrix:
         toolchain:
-          - 1.87.0
+          - 1.82.0
           - stable
         os:
           - ubuntu-latest