feat(base-template): compile-time .jt template library

[deer]

• Adds base-template, a lightweight runtime library providing Template<O>, HtmlOut (with XSS-safe HTML escaping), and TextOut
• Adds base-template-processor, an annotation processor that compiles .jt template files in src/main/jt/ into Java records at build time
• Adds base-template-test, an integration test module that exercises the full parse → generate → compile → render pipeline