Document unlisted env vars + write down all env vars in a single file

[Human9000-bit]

Only 32 of the 136 environment variables the workspace reads were
documented, spread across four sections that had drifted apart. Add
docs/env-vars.md covering all 158 (including CI scripts), grouped by
area with defaults taken from the call sites rather than the old docs.

Two things the previous docs never stated and that bite in practice:
most knobs are presence-checked, so YSCV_REORDER_FUSION_OFF=0 still
disables the pass; and every hot-path knob is OnceLock-cached on first
read, so setting one mid-process does nothing.

Three variables were documented but no longer exist in the code
(YSCV_FUSED_DW_PW_STREAM_MT, YSCV_FUSED_DW_PW_STREAM_PADDED_OFF,
YSCV_MPS_DEBUG), and YSCV_AARCH64_LOW_K_BLOCKED_MIN_WORK_FMAS was
listed with a default of 500000 where the code uses 1048576. The claim
that x86_64 enables padded DW->PW streaming by default was also wrong:
it is off on every target.

Keep the existing curated "most used" lists in place and repoint their
canonical-reference links here; onnx-cpu-kernels.md keeps its A/B table
since the tracker reproduction commands below it use those vars.