Use Tile for map cells and their accessor

[genixpro]

Replace the French map-cell name Case with Tile, rename getCase to getTile and the map's cases storage to tiles, and update the editor's member pointer and current test harnesses. This is the map terminology portion of Stéphane Magnenat's #85, adapted to current master.

All string literals remain unchanged, including the historical cases save section. No data layout, algorithms, or gameplay changes are intended.

Validation:

• Lexical comparison confirms only the documented identifier substitutions outside comments, with every string/character literal preserved.
• macOS release client and server builds pass with both replacement patches applied together to master 88934ec.
• All 16 existing test executables pass on that combined code, including TestsRunner's 170 tests and replay/network/text-serialization tests.
• G2, playground-8numbi, gd-archipelago, and gd-bigarena-long: 15,000 ticks each, before/after replay bytes and every per-tick checksum identical (60,000 ticks per version).
• Each PR also runs its own Linux/Windows CI; see checks below.

Companion PR: #200

This replaces part of #85. The complementary spelling/comment patch and Kyle's already-landed cb7856e cover the selected spelling cleanup; the original broad casing-only sweep is intentionally omitted.

[Giszmo]

Reviewed the full diff and PR head: pure identifier rename (Case→Tile, getCase→getTile, cases→tiles), +151/-151 exact wash, no leftover references, no smuggled behavior changes, save-format section-name literal ("cases") preserved for backward compat, CI green on linux 22.04/24.04 and Windows. Approving.

Non-blocking nit: test/WinningConditionsHarness.cpp had an unrelated local Case struct used for prestige test scenarios that got swept into the same mechanical rename and is now confusingly named Tile/tiles[] despite having nothing to do with map tiles. Worth a follow-up rename (e.g. PrestigeCase), not blocking this PR.

[genixpro]

Merging approved head d91ce5a1df0f9e264158823987ebda0ffe4573b1 with Bradley's explicit authorization and Giszmo's approval of this exact commit.

Final verification checked unchanged head, approval and successful Ubuntu 22.04, Ubuntu 24.04 and Windows CI: https://github.com/Globulation2/glob2/actions/runs/34183000870 . Giszmo independently reviewed the full diff as identifier-only renaming, preserving the serialized "cases" section literal. The PR's existing validation record also reports macOS client/server builds, 16 test executables and 60,000 ticks per version with identical replay bytes/checksums; those runs were not repeated during this merge check.

No gameplay, data-layout or save-format changes are intended. The reviewer marked the unrelated prestige-test type naming nit non-blocking.