Add Pokemon Crystal as a sixth supported version

Crystal boots from a user-supplied ROM, imports a full cache and is playable:
copyright, the Crystal intro movie, the animated title, gender select, Oak,
and out into Johto. 122 of the cart's 169 script specials are implemented.

Import and data
- tools/make_crystal_manifest.py derives the manifest by importing
  make_gold_manifest as a library, with three additive keyword seams. Gold and
  Silver still regenerate byte-identical, which is the standing requirement for
  touching that generator.
- crystal_symbol_deltas.py and crystal_movie_symbols.py carry the symbol delta:
  Crystal renames the credits mons, splits the trainer card, Pokegear and
  pack-pal blocks by gender, and replaces the intro and title outright.
- Crystal-only manifest keys: engineFlagOrder (162 flags to Gold's 93, so the
  badge block sits one higher) and unownCharmap (the main charmap parser stops
  at the first newcharmap so the two cannot contaminate each other).

Extractor
- RomExtractorGen2 becomes three-edition. Crystal corrections: PAL_MAP_BANK
  0x13, a flat PICS_FIX pic bank, audio bank 0x5e, the mapSongs id-100 hole,
  seven NPC trades, a TradeTexts stride of 8, the five Crystal tileset anim
  steps with per-row degrade, and the column-major trainer card portraits.
- New: animated front sprites (frames, bitmasks, play and idle scripts), the
  Battle Tower roster, Kris assets, Mobile System GB art, and the Crystal
  intro and title via src/import/CrystalMovie.lua.

Engine
- GameVersion gains engine(id) and fixes(id). Gold and Silver keep their
  original bugs where the bug is not hardware dependent; Crystal gets the fixes
  Crystal shipped: Lucky Number boxes 10-14, surfing onto an NPC, and the
  Reflect and Light Screen defence overflow.
- Crystal story: Suicune and Eusine, Celebi behind the GS Ball flag, the Ruins
  of Alph chambers, Buena, the Move Tutor, the Poke Seer, and the Battle Tower
  including the wInBattleTowerBattle badge-boost guard.
- Kris and the gender flag, animated fronts in battle and the summary screen,
  and mon caught data.

Verification
- Every extracted asset is pixel-compared against pret's own source PNGs.
- Gold caches are byte-identical before and after, file for file.
- New Crystal suites plus a T2 Gen 2 tier; the full suite passes.

CONTRIBUTING-mods.md

@@ -140,8 +140,8 @@ ship text.
 
 ### 4. `games` (and the legacy `gen2compat`)
 
-Pokemon Gold and Silver are Gen 2, and they run their own battle engine,
-overworld, script VM and save format. The mod API is shared across both
+Pokemon Gold, Silver and Crystal are Gen 2, and they run their own battle
+engine, overworld, script VM and save format. The mod API is shared across both
 generations (same hook names, same event names, same registry names) but Gen 2
 cannot serve all of it yet, so it is opt-in. Say which games the mod is for:
 
@@ -150,19 +150,20 @@ cannot serve all of it yet, so it is opt-in. Say which games the mod is for:
 ```
 
 Each entry is a version id (`"red"`, `"blue"`, `"yellow"`, `"gold"`,
-`"silver"`), a generation (`"gen1"`, `"gen2"`) or `"all"`;
+`"silver"`, `"crystal"`), a generation (`"gen1"`, `"gen2"`) or `"all"`;
 `src/mods/ModTargets.lua` resolves them off `GameVersion.ORDER` so nothing
-restates the game list. `python3 tools/modkit.py scaffold my_mod --games
-gen1,gen2` writes the key for you. The mod still installs to one directory,
-`mods/<id>/`, shared by every game -- targeting is declared, never filed.
+restates the game list, which is why `"gen2"` covers Crystal as well as Gold
+and Silver. `python3 tools/modkit.py scaffold my_mod --games gen1,gen2` writes
+the key for you. The mod still installs to one directory, `mods/<id>/`, shared
+by every game -- targeting is declared, never filed.
 
 Absent means Gen 1 only, which is what every mod written before the key existed
 was tested as. `"gen2compat": true` is the legacy spelling, still accepted and
 purely additive (it *adds* the Gen 2 games), so no manifest can lose a game it
-already ran on. On a Gold or Silver boot a mod claiming no Gen 2 game is not
-loaded at all: the manager lists it as `ENABLED (NOT THIS GAME)` and says why,
-because a mod that half-applies reads as a broken mod. Claim Gen 2 once you
-have actually run your mod on Gold or Silver.
+already ran on. On a Gen 2 boot a mod claiming no Gen 2 game is not loaded at
+all: the manager lists it as `ENABLED (NOT THIS GAME)` and says why, because a
+mod that half-applies reads as a broken mod. Claim Gen 2 once you have actually
+run your mod on Gold, Silver or Crystal.
 
 Every token is enforced, per game: the loader gates on the same
 `ModTargets.supports` answer both mod surfaces draw, so `"games": ["blue"]`
@@ -172,10 +173,11 @@ manifest with neither key still covers every Gen 1 game, so nothing written
 before the key existed changes behavior; list both generations or say `"all"`
 when you mean everywhere.
 
-`docs/mod-api-gen2-compat.md` is the compatibility matrix: what works on Gold
-and Silver today (40 of the 46 registries, 40 event and 44 hook names shared with Gen 1,
-and 24 Gen 2-only ones), which registries have no Gen 2 home and drop their
-writes with a report, and which hooks and events are still to come.
+`docs/mod-api-gen2-compat.md` is the compatibility matrix: what works on Gold,
+Silver and Crystal today (40 of the 46 registries, 40 event and 44 hook names
+shared with Gen 1, and 24 Gen 2-only ones), which registries have no Gen 2 home
+and drop their writes with a report, and which hooks and events are still to
+come.
 `docs/preparing-your-mod-for-gen2.md` is the step-by-step migration guide for a
 Gen 1 mod, and it is the one to start from.