fix(lang-core): keep multi-line ternaries intact in mergeStatements

[serhiizghama]

Closes #821.

A statement written as a multi-line ternary survives parse(), but mergeStatements() was silently dropping both branches even when the patch never touched that statement:

a = $ok
  ? Title("Yes")
  : Title("No")

becomes a = $ok after merging an unrelated patch.

The cause is the third statement splitter. split() in statements.ts and scanNewCompleted() in parser.ts both know that a ? / : continuation line belongs to the same statement (they track ternaryDepth and peek past the newline), but mergeStatements() re-splits the existing program with splitStatementSource() in merge.ts, which only tracked bracket depth and broke on every depth-0 newline. The ? ... / : ... fragments then failed the Ident = expression shape check and were discarded without an error.

Went with direction 1 from the issue: gave splitStatementSource() the same ternary handling — track ternary depth at bracket depth 0, and before treating a depth-0 newline as a boundary, skip it if we are mid-ternary or the next meaningful character is a ?. Didn't unify onto split() (direction 2) because merge needs the raw source slices to round-trip the statements, and the token splitter throws that text away.

Added two tests next to the existing merge cases: the issue repro (untouched ternary stays intact) and the inverse (a patch targeting the ternary still replaces it). The first fails on the current splitter. Full lang-core suite is green, tsc/eslint/prettier clean.