versus: accrue per-player stats per chapter, not per whole match (0.7.0)

[draxios]

Versus stats now accrue per chapter, not per whole match

In versus a match is a whole campaign and almost never finishes — players
leave near the end. But player_versus_stats (the per-player versus record a
profile/leaderboard reads) only rolled up at match close, so in practice it
stayed empty, match_maps mostly sat incomplete, and every match_rounds
row was round 1 (round 2 never recorded). Diagnosed live: 85 match_rounds, 412
player_round_stats rows, 0 player_versus_stats rows.

What changed

• Per-chapter rollup. player_versus_stats now rolls up at the close of each
  chapter (a map's two halves — "play Survivor and Infected once"), which
  reliably completes. It runs inside the round-2 DB transaction so the rollup sees
  both halves' player_round_stats atomically. A completed chapter credits every
  player immediately, even if the match is later abandoned. maps_won/maps_lost
  are the win/loss record (one per chapter); streaks count consecutive chapters.
  Match close only touches matches_* (no double count).
• Round 2 is recorded. Half detection moved from the unreliable
  versus_round_start/is_secondary_round (stays false here → Duplicate entry 'N-1') to counting the engine round_start with idempotency guards.
• First-chapter drop fixed. The first chapter of every match was lost to an
  async race (OpenMatchMap() ran while g_MatchId was still 0); it's now opened
  from the OnMatchInserted callback.
• Chapter winner from plugin Survivor points (the engine distance netprop
  reads 0 on this build — an engine-accurate winner is a documented follow-up).
• Robustness: HookEvent → HookEventEx (a build missing an optional event
  like entered_checkpoint no longer aborts OnPluginStart — this was blocking
  the plugin from loading on the prod build), and GetAdminTopMenu() guarded with
  LibraryExists("adminmenu").

Verification

• Compiles clean on SM 1.12 (no errors); loads Status: running on prod.
• Reviewed by a 4-lens adversarial pass (compile / SQL / state-machine /
  semantics) — its findings (first-chapter race, engine-score index, round W/L
  over-count, event-ordering) are addressed here.
• Deployed to Bizzy's Dugout (prod, that server only) to bake. Per-chapter
  recording needs a live versus match to fully validate — an empty server can't
  reproduce team flips. Once a real match confirms player_versus_stats /
  match_maps populate correctly, the compiled plugin will be vendored into the
  bizzymod-campaign monorepo PR and both merged.