Add Middleware API; untrack Main.java and local artifacts

- Add Middleware.java: @FunctionalInterface with Middleware.of() chain
  composition and andThen() helper; zero allocations on hot-path
- Remove flash/Main.java from versioning (scratch/test entrypoint)
- Update .gitignore: exclude flash-bench/, nuxt-shadcn-dashboard/,
  /dev/, /docs/, Main.java, *.text — all root-level local artifacts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -38,4 +38,13 @@ build/
 ### Mac OS ###
 .DS_Store
 
-CLAUDE.md
+CLAUDE.md
+
+### Local / scratch ###
+flash/src/main/java/dev/relism/Main.java
+flash-bench/
+nuxt-shadcn-dashboard/
+/dev/
+/docs/
+jmh-result.text
+*.text