feat(gen2): read applyShare's announce argument in battle.exp_award

battle.exp_award hands a mod ctx.applyShare(mon, split, announce) on both
generations, and on Gen 1 the third argument decides whether the mon's
GainedText box is printed -- which is how a mod paying the whole party
prints ONE summary line instead of a box per recipient.  Gold accepted
the argument and ignored it, so the same mod source printed one line on
Red and one per party member on Gold.

The Exp Share mod is the live case: it declares games gen1+gen2 and its
description promises "a single shared-exp line instead of one message per
Pokemon", passing true for the fighters and nil for the bench exactly as
the Gen 1 seam asks.  On Gold every nil call announced anyway, so a
five-mon party turned every KO into six boxes.  There was no mod-side
fix: the emit sits behind no hook, and the argument meaning "quietly" was
discarded.

Gold now reads it, and ONLY when it is actually passed -- by argument
count, not by value.  select("#", ...) counts an explicit nil, so
applyShare(mon, split) is distinguishable from applyShare(mon, split,
nil); the first is a Gen 2-era call written against a seam that always
announced and keeps announcing, the second is a deliberate "pay this one
quietly" and is now silent on both games.  No mod that exists today
changes behaviour, and a mod that passes the argument gets parity.

Only the { kind = "experience" } event is affected.  A silent award is
still a whole award: exp, stat exp, battle.exp_gained, "grew to level",
learned moves and the forget prompt are untouched, in the same order.
giveExperiencePass takes a sixth `silent` parameter that defaults to
announcing, so both of the cart's own passes are unchanged.

RFC: docs/rfcs/0012-gen2-exp-award-announce.md
Docs: docs/mod-api-gen2-compat.md gains the reading and the residual
      omitted-argument difference beside the existing payload note.
Tests: tests/gen2_exp_share_test.lua grows the two Route B tests -- the
       no-mod parity case and the seam driven through hooks:wrap -- and
       its 23 existing checks are unchanged.

docs/mod-api-gen2-compat.md

@@ -550,6 +550,17 @@ gains a field instead of the name gaining a prefix.
   screen has no `.data` field, so the Gen 2 site **adds** `ctx.data` beside the
   Gen 1 keys. A mod that calls `nextFn` is unaffected; one that reaches through
   `ctx.battle.data` instead gets nil on Gold.
+  `battle.exp_award`'s `ctx.applyShare(mon, split, announce)` reads its third
+  argument on both generations: truthy prints the mon's GainedText, falsy pays
+  it silently, so one mod source can print a single summary line for a
+  party-wide award instead of a box per recipient. Gold honours it **only when
+  it is passed**, by argument count -- `applyShare(mon, split)` was written
+  against a seam that always announced on Gold and keeps announcing there,
+  while `applyShare(mon, split, nil)` is silent on both. Pass the argument
+  explicitly and the two generations agree; omit it and Gen 1 stays silent
+  where Gold speaks. Only the line is affected: the exp, the stat exp,
+  `battle.exp_gained`, the level-up line, learned moves and the forget prompt
+  happen either way.
 - *The catch and the evolution:* `pokemon.caught`, `pokemon.evolved`; hook
   `evolution.check`. `src/ui/gen2/BattleState.lua:pushCaught` emits
   `pokemon.caught` once the mon is in the party or the box, and