feat(engine): variable ceilings + window gaps (upper wall slices)

[Chemaclass]

Phase 3 of the variable floor heights epic #375. Tracking issue - this was the largest phase, now decomposed into shippable sub-issues per its own "split when picked up" note. Each is behavior-neutral on existing (flat-ceiling) levels and merges independently.

Motivation

Phase 1 raises floors but keeps the ceiling fixed at z=1. Real DOOM rooms also vary ceiling height: lowered ceilings, doorframe lintels, and window gaps you can see (and shoot) through but not walk through. A cell with raised fz and lowered cz becomes a window gap.

Sub-issues (order)

• feat(map): per-cell ceiling height cz + :pcz hot array #385 map: per-cell ceiling height cz + :pcz hot array (mirror of feat(map): per-cell floor-height data (fz) + :pfz hot array #368)
• feat(engine): upper-span cast where ceiling drops #386 engine: upper-span cast where ceiling drops (mirror of feat(engine): multi-span cast — DDA continues past floor risers #369)
• feat(render): upper wall-slice painter + ceiling-cast row distance #387 render: upper wall-slice painter + ceiling-cast row distance (mirror of feat(render): multi-span column painter — risers, tier tops, row clip #370)
• feat(physics+combat): window-gap passability + shoot-through #388 physics + combat: window-gap passability + shoot-through

Order: 385 -> 386 -> 387 -> 388. Data/cast/render/physics-combat land separately.

Global acceptance

• Golden-frame + unit coverage per sub-step.
• /perf-bench gate as in feat(render): multi-span column painter — risers, tier tops, row clip #370.
• Update docs/raycaster.md, docs/rendering.md, docs/map.md.

Dependencies

#370 (span painter). Independent of #372/#373.

[Chemaclass]

Phase 3 complete. All four sub-issues merged, each behaviour-neutral on flat-ceiling levels:

• feat(map): per-cell ceiling height cz + :pcz hot array #385 map: per-cell ceiling cz + :pcz hot array (PR feat(map): per-cell ceiling height cz + :pcz hot array #389)
• feat(engine): upper-span cast where ceiling drops #386 engine: upper-span :uspans cast where the ceiling drops (PR feat(engine): upper-span cast where ceiling drops #391)
• feat(render): upper wall-slice painter + ceiling-cast row distance #387 render: upper wall-slice painter + ceiling-top-row-dist (PR feat(render): upper wall-slice painter for lowered ceilings #392)
• feat(physics+combat): window-gap passability + shoot-through #388 physics + combat: window-gap headroom passability (headroom-clears?) + shoot-through (cast-shot-dist) (PR feat(physics+combat): window-gap passability + shoot-through #393)

Ceilings are now data -> cast -> render -> physics/combat end to end. Docs updated (docs/raycaster.md, docs/rendering.md, docs/map.md, docs/combat.md). Enemy chase headroom parity deferred (no shipped lowered-ceiling content), consistent with the epic's other deferred enemy-elevation work. Closing the tracker.