refactor: extract LpStats cluster from the SolveStats god-object

[rasros]

What changed

SolveStats (and its SolveStatsSink) defined every stat in three places — the immutable snapshot field, the sink accumulator, and the mergedWith merge line — with the 12-field LP cluster dominating that triple maintenance. Group the LP stats into their own LpStats / LpStatsSink pair that owns its snapshot and merge:

• New result/LpStats.kt: LpStats (immutable, with mergedWith) + LpStatsSink (accumulators + observe* methods + snapshot()).
• SolveStats holds val lp: LpStats; its mergedWith/snapshot() delegate one line to the cluster instead of twelve.
• SolveStatsSink holds val lp: LpStatsSink; the LP observe* methods moved onto it (sink.observeLpSolve() becomes sink.lp.observeSolve()), and the shared ZERO_COUNT / NO_MAX / naNDeferring helpers were promoted to package-level so both records use them.
• Dropped the unused @Serializable annotations (nothing serializes these types).

Readers move stats.lpSolves.sum to stats.lp.solves.sum (~44 sites, mostly tests + CliStats); observe* callers move to sink.lp.* (~17 sites in backtrack/lp).

Why

Adding or changing one LP counter meant four scattered edits in a 515-line file. Now it's one ~60-line cluster where the field, its merge, its accumulator, and its snapshot are colocated — and SolveStats becomes composition + delegation. First cluster of several; search / conflict-analysis / scheduling / local-search can follow the same recipe.

Testing

Behavior-preserving: the emitted -s / %%%mzn-stat key strings (lpSolves, lpPruned, ...) are unchanged — only the in-tree field paths moved. Full check lintDocs green across JVM + native + wasm.