Thanks to Goldman, MNTA, aubbap, sharckry and Pludicrousman: this update comes entirely from your feedback.

⚡ 2x and 4x speed

At increased speed your towers really did fire less often, and a projectile could overshoot its target without ever hitting it. Enemies, meanwhile, genuinely moved four times faster. Playing at 4x was therefore harder than at 1x with nothing to tell you, and flying enemies became nearly untouchable. The simulation now advances in fixed steps: for the same amount of game time, 1x, 2x and 4x give exactly the same result.

♾ Endless Run

The Endless Run now has a summit: it ends at wave 1337, its final milestone, on a dedicated screen. The monthly leaderboard rewards a build, not a night spent leaving the game running.
The FEARLESS WARRIORS milestone no longer makes enemies completely immune to slow and freeze: they strongly resist both, and gain health in exchange. MAELSTROM, ETERNAL CRYSTAL and control towers stay useful deep into a run.
The Endless Run lobby could become permanently inaccessible. Fixed, including for players already locked out.
Kill streaks no longer break when only bosses remain on the map.
Leaving and resuming a run reset every shop price and emptied the active bonus panel. Both are now preserved.
Bonus percentages eventually displayed in scientific notation after a very large number of purchases.

⚔️ Balancing

The Sniper's base damage is reduced. It is the only tower whose range covers the whole map: it fires with no placement constraint and was worth far more than its cost suggested. The Master Marksman relic goes from +50% to +40%.

🗺 Campaign

Chapters completed in Infernal Shop or One Life now carry a badge on the campaign map.
Campaign best scores are kept from one session to the next again. The My Chapters tab showed times, but never scores.

🏪 Shop

Transcendence and Amplified Resonance are no longer offered when no tower on the board can benefit from them. Buying one simply refunded you in silence, with no explanation. A message now says so.

👁️ Comfort

On small screens, and on desktops scaled to 150% or more, the window now fits the space actually available and the title screen is no longer cut off.
The score tooltip explains the role of combos and elemental synergies, and why two runs built with the same towers do not give the same score.
Under heavy load, lightning projectiles turned white.