Drop SemanticDB output from semanticdb-javac (split 1/2)

[jupblb]

Part 1 of a 2-PR split of #891.

What this PR does

Replaces the legacy SemanticDB → SCIP conversion pipeline on the Java side with a direct, in-plugin SCIP shard emission flow. The Kotlin plugin keeps emitting SemanticDB for now and gains dual-emission so the new SCIP aggregator can read its data too. Part 2 (#TODO) drops Kotlin SemanticDB emission and removes the SemanticDB protobuf machinery entirely.

Pipeline before vs after this PR

Before:
  javac/kotlinc + plugin  ──►  *.semanticdb files  ──►  scip-semanticdb aggregator  ──►  index.scip

After:
  javac + plugin     ──►  *.scip shards  ─────┐
                                              ├─►  ScipShardAggregator  ──►  index.scip
  kotlinc + plugin   ──►  *.scip + *.semanticdb (dual)

Commits

15 commits, grouped:

• M1-M8 + cleanup (9 commits): SCIP shard infrastructure — ScipShardWalker, SymbolRewriter, ScipVisitor (forked from SemanticdbVisitor), ScipSignatureFormatter, ScipShardAggregator, the opt-in --use-scip-shards CLI flag, snapshot stabilization, and making compiler-side SCIP emission the Java plugin default.
• K1-K5 (5 commits): Kotlin SCIP infra + dual-emission + aggregator robustness fixes + CLI default flip to shard consumption + snapshot regen. (Necessary in this PR so the aggregator's now-default SCIP path still picks up Kotlin data while Kotlin SemanticDB output remains.)
• D1-D2 (2 commits): ScipVisitor no longer references SemanticDB protobuf types; semanticdb-javac emits only SCIP shards.

What is NOT in this PR (deferred to #TODO)

• D3: semanticdb-kotlinc emits only SCIP shards
• D4: aggregator drops the SemanticDB fallback path
• D5: drop the SemanticDB protobuf + dead builders
• D6: scrub SemanticDB wording from docs and help text
• 4 cleanup commits that simplify code only reachable after Kotlin's drop