Single-pass stereo: fix the remaining visual bugs and enable by default

[philpax]

Problem

Single-pass stereo (merged in #45, default-off) is promising at reducing the
drops below the target framerate, but it isn't ready to enable by default:
several visual bugs remain when it's on.

Known so far:

• Terrain tile transition discontinuities.
• The right eye renders incorrectly in-headset.
• Water is completely broken.
• Other assorted oddities as testing continues.

This is a deliberately loosely-scoped epic: symptoms may split into individual
issues as they get root-caused, and this closes if/when SPS becomes the
default.

Notes

The per-render-block record and the documented known gaps live in
docs/mod/stereo/single-pass-stereo.md and
docs/mod/stereo/single-pass-render-blocks.md; the ~25 sub-flags in
stereo.single_pass.* each name the artifact they address, and some are still
blind-implemented (e.g. occluder). The legacy-water intercept's
reachability still needs one in-headset session with the decline diagnostic.