ci(foundry): add forge build cache + 45min timeout (#123)

Foundry CI on main has been dying with exit 143 (SIGTERM) at the
'Run Forge build' step around 17min — symptoms consistent with
the runner OOM-killing solc on a 220-file via-IR build.

Adds an actions/cache@v4 step pinned to foundry.toml + soldeer.lock
+ src/test/script/dependencies hashes, restoring incremental
forge build state across runs. Also pins a 45min timeout so we get
a clear timeout signal instead of a generic SIGTERM if a future
build genuinely runs over.

If this still OOMs after the cache lands, next step is to switch
to a larger runner (ubuntu-latest-l) once that's enabled for the org.

Author: drewstone

.github/workflows/foundry.yml

@@ -23,6 +23,7 @@ jobs:
 
     name: Foundry project
     runs-on: ubuntu-latest
+    timeout-minutes: 45
     outputs:
       foundry_changed: ${{ steps.changes.outputs.foundry_changed }}
     steps:
@@ -31,6 +32,17 @@ jobs:
           fetch-depth: 0
           submodules: recursive
 
+      - name: Cache Foundry build artifacts
+        if: steps.changes.outputs.foundry_changed != 'false'
+        uses: actions/cache@v4
+        with:
+          path: |
+            cache
+            out
+          key: foundry-${{ runner.os }}-${{ hashFiles('foundry.toml', 'soldeer.lock', 'remappings.txt', 'src/**', 'test/**', 'script/**', 'dependencies/**') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
+
       - name: Detect Foundry-relevant changes
         id: changes
         run: |