docs: reconcile the GDD, README and TODO with the code

[vetr0s]

The GDD is the design source of truth and it had drifted. Every claim below was verified against the code, not guessed at. Docs only, no code changes, so this is independent of #16 and #17 and can merge in any order.

GDD (docs/VERSION 0.4.0 -> 0.5.0, PDF regenerated)

claim	truth
dead-end ore falls over FALL_TICKS via item_begin_fall / world_update_falls	none of the three symbols exist. The sim kills the ore the tick it dead-ends and emits a World_Event; the renderer turns that into a real-time Effect. That sim/visual split was a deliberate decision the doc never recorded, so it is written down now
real sprites ship for the machines and the ore	assets/sprites/ is empty. The pipeline shipped (a PNG dropped in is baked into the binary, and the ore already falls back to the procedural diamond). Everything draws as a block today
the region starts at 2x2	it starts at 4x4 and caps at 128x128. The 2x2 example was dead on its own terms: the starter chain is 3 cells wide
money buys tiers along "three axes today", including belts	two. Belt speed is still the one global sim tick, so a belt cannot have a tier until the per-entity cadence rework lands
four object types (color, visual design, tick lists)	five, since the splitter. The GDD already described the splitter correctly elsewhere; these lists were never updated
save format is "binary or simple text, human-editable", with "periodic auto-save"	a versioned binary blob, and there is no auto-save
keys 1-5, Escape quits	1-6, plus T, E, -/=, K, L, backtick. Escape pauses; it only quits from the title screen
shop shows cost on hover; ghost is green/red	costs are always on screen (greyed when unaffordable), and the ghost has a third state (affordable-but-broke)
Milestone 3 in progress; Milestone 4 lists save/load + culling as future	all three shipped

Also recorded the two invariants the audit made real: every running total saturates rather than wrapping i64, and "once per distinct upgrader" is a per-ore bitmap whose width is the cap on upgraders in the world.

README

Esc pauses (it said Quit). The controls table was missing 6, T, E, -/=, K and L. The Entity_Upgrader and World samples no longer matched the structs (the upgrader one would not even compile). The source tree omitted effects, sprites, menu and save. "Everything else lives behind the debug overlay" was false. And "Playing with it" never mentioned money, the shop, expansion, the splitter, save/load or the menus, while telling you to build a loop that, without a splitter, jams.

TODO / CHANGELOG

UPGRADER_MULT never existed; the overlay is backtick, not F3; the GDD is v0.5.0, not v0.2.0; interactive UI, the menu's Continue and viewport culling are done; and the Docs item described an SDL3 stack fix that had already happened. The CHANGELOG's one factual error (F3) is corrected, which the file already contradicted a section later.

Verification

Grepped the docs for every stale symbol and number and checked each against the source: PLAYABLE_START_SIDE = 4 vs "starts at 4x4", PLAYABLE_MAX_SIDE = 128 vs "128x128 cap", zero PNGs in assets/sprites/. typst compile clean. Game VERSION deliberately untouched at 0.7.0; only docs/VERSION (the GDD's own) moves.