feat(binaural): grow the reflection room to contain the scene (S2-16)

[mgth]

Summary

Series 2, lot 3, S2-16 — automatic room floor.

The image-source room was whatever the user configured; a scene larger than it (any unit_scale_m past 1 with the default 4 × 5 × 2.7 m room) had every boundary object pulled back to the wall before mirroring — the reflections of the wall rather than of the object, and a copy of the direct sound 0.1 m behind it from the nearest wall. The doc asked the user to size the room by hand.

• reflections::room_containing_scene(room, unit_scale_m): each half-extent floored at unit_scale_m + ROOM_FLOOR_MARGIN_M (0.35 m), capped at 20 m. Applied once per frame in render_frame; per channel and per sample nothing changes.
• The plan wrote the floor as √3·unit_scale_m + margin (the scene's circumscribed sphere). The walls are world-fixed and the scene does not rotate with the head, so per-axis containment of the ADM cube is the exact condition; √3 would have oversized every axis by 73 %.
• 0.35 m margin: a boundary object's nearest image trails the direct sound by 0.7 m (≈2 ms), and the default 2.7 m ceiling is exactly the floor at unit scale 1 — the default room is unchanged at unit scale 1. At scale 3 every axis becomes 6.7 m, the 6–8 m the doc used to recommend.
• BINAURAL.md: the room extents are documented as minimums; the Scale tip rewritten.

Tests

• room_floor_contains_the_scene (reflections.rs): default unchanged at scale 1, 6.7 m at scale 3, generous room kept, 20 m cap, cube corners never clamped.
• room_grows_to_contain_the_scene (mod.rs): at scale 3 in the default room, the boundary source's nearest reflection lands ≈98 samples (0.7 m) after the direct sound, nothing before.
• reflections_of_an_outside_source_do_not_coincide_with_the_direct moved to unit scale 12 (past the cap) so the clamp path stays covered.

cargo test -p renderer --lib: 383 passed, 2 ignored. Golden unchanged (reflections off).

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