cli: validate computed Java/Kotlin/Groovy package name before generating (reserved words, leading-digit segments)

[marioserrano09]

Problem

dynamia new doesn't validate that the computed Java/Kotlin/Groovy base package
(groupId + '.' + artifactId.replace(/-/g, '.')) is actually a valid package name for the
target language before generating. Two concrete failure modes found while smoke-testing the
26.9.0 release:

1. Reserved words: --artifact-id final-java (groupId com.acme) computes base package
   com.acme.final.java -- final is a reserved word in Java/Groovy, so the generated project
   fails to compile with cryptic errors like <identifier> expected. Kotlin is unaffected since
   it treats final as a soft/contextual keyword.
2. Leading-digit segments: --artifact-id smoke-java-2 computes base package
   com.acme.smoke.java.2 -- a package segment can't start with a digit in Java/Kotlin/Groovy.
   Fails with ';' expected (Java) or a Groovy stub-generation parse error, again with no hint
   about the actual cause.

Impact

A user picks a project name that happens to collide with a language keyword or produces an
invalid package segment, and gets a wall of mvn/compiler errors with no indication the CLI
itself is the source of the problem.

Proposed fix

In platform/packages/cli/src/commands/new.ts (or a small helper in utils/), validate the
computed base package before cloning/generating:

• Each dot-separated segment must not be empty, must start with a letter or underscore (not a
  digit), and must not be a reserved word in the target language (Java reserved words are a
  superset that also covers Groovy; Kotlin's hard keywords are a smaller list -- soft keywords
  like final are fine to allow for Kotlin specifically).
• On failure, show a clear message before cloning anything, e.g. --artifact-id "final-java"
  would produce an invalid package segment "final" (Java reserved word) -- pick a different
  artifact ID, and re-prompt (interactive) or exit with DT-COMMAND-003` (non-interactive).

Repro: dynamia new --name x --scaffold backend --backend-lang java --group-id com.acme --artifact-id final-java --yes --no-git, then cd x/backend && ./mvnw compile.