chore: apply initial spotless formatting

[wombatu-kun]

Summary

Stacked on top of #23 (build: add spotless, checkstyle, and enforcer plugin configuration). Marked as draft until #23 is merged — at that
point GitHub will recompute this PR's diff to show only the formatting
commits. Until then the diff includes both PRs' changes.

• Run mvn spotless:apply on every file under src/main/java/**/*.java
  and src/test/java/**/*.java (31 files). Effect: google-java-format
  layout (2-space indent, 100-char line length), import order
  org.apache.flink,,javax,java,#, removed unused imports, license
  header rewritten to the short Apache 2.0 form.
• Manually fix the 6 Checkstyle violations Spotless does not handle:
  • LanceInputFormat.java:295 — drop public on the LanceSplitAssigner
    constructor (RedundantModifier — the enclosing class is private).
  • LanceIndexBuilder.java:325 — wrap the Builder.m(int m) HNSW
    builder method in // checkstyle.off: MethodName /
    // checkstyle.on: MethodName. The single-letter M is the
    standard HNSW parameter; renaming would break public API.
  • LanceCatalog.java:512 — split a 110-char log message string.
  • LanceDynamicTableSource.java:280–283 — add braces to the
    if/else if operator-flip chain (NeedBraces).
• Fix .editorconfig: Java indent_size 4 → 2 to match the actual
  google-java-format output Spotless writes. Without this, IDEs that
  honour .editorconfig would re-indent files to 4-space and fail
  mvn spotless:check on every save.
• Bind spotless-maven-plugin and maven-checkstyle-plugin to the
  validate Maven phase (goal check). Now mvn compile (and any
  later phase) automatically enforces both, so style regressions
  fail the build.

The reformatting commit is purely mechanical — no behavioural changes,
no test changes.