fix(parser-native): restore trailing return in bare switch case bodies (#597)

[cs01]

Summary

Fixes #597 — the regression that blocked all stdlib additions past 8 count. Unblocks #585 (net TCP module) and every future stdlib registration.

User-facing effect

• Compiler now correctly compiles itself when it has 8+ `registerStdlib()` calls. Previously failed with a false-positive `checkMissingReturns` error on `transformExpression`.
• stdlib module count is no longer artificially capped.

Root cause

Parser-native's switch_case iteration was silently dropping the trailing `return` statement when a bare case body had the shape `[lexical_declaration, if_no_else, return]`. Tree-sitter reports fewer `namedChildCount` siblings than expected for this specific pattern, so the loop at `transformer.ts:2297` never visits the final `return`. The case body ended up as `[var_decl, if_no_else]` — no path returns — and `checkMissingReturns` correctly flagged it.

Only `case "unary_expression":` at `transformer.ts:528` had this 3-statement bare shape in the compiler source, so it was the lone trigger. Other bare case bodies had at most 2 statements (`[var_decl, return]`) and weren't affected.

Fix (two parts)

1. `transformStatement` handles `statement_block`: was `return null`, now calls `transformStatementBlock` to preserve the block's contents. Without this, any `{}`-wrapped case body would silently drop.
2. `case "unary_expression":` body wrapped in `{}`: forces tree-sitter to produce a single `statement_block` child, which now traverses correctly, preserving all three inner statements.

Followup

• Deeper root cause (tree-sitter or parser-native case-body iteration dropping bare siblings) is not addressed here. Captured in regression: checkMissingReturns false-positives on transformExpression when chad-native.ts has 8+ registerStdlib calls (introduced in #583) #597 comments.
• semantic: reject bare case-declarations without a block #601 proposes a semantic pass to reject bare case-declarations at compile time — would prevent future regressions of this class.

Test plan

• `npm run verify` — full tests + 3-stage self-host green locally (with 8+ stdlibs — the repro threshold)
• CI green