Fix SQtqmse codebook scale: fold 1/sqrt(d) into Lloyd-Max table

[mnorris11]

Summary:
SQtqmse reconstructions have inflated norms and badly degraded recall since
D102408184. Reported as a 1.14.2 -> 1.14.3 regression: at d=64, median decoded
norm of a unit-norm input went 0.9965 -> 1.1503 (+15%), while SQtqmse8
looked "essentially unchanged".

Fixes #5317

Root cause

D102408184 replaced the tqmse codebook construction with hardcoded Lloyd-Max
tables:

-    scalar_quantizer::train_TurboQuantMSE(d, 4, trained);
+    populate_lloyd_max_trained(4, trained);

The old codebook was built for one coordinate of a unit-norm vector in R^d, so
its centroids scaled as 1/sqrt(d). The new tables are optimal for N(0, 1)
and do not depend on d at all.

That is correct for _eden and _tq, which rescale each vector to unit
variance before lookup. Plain tqmse does not: it encodes raw unit-norm
vectors, whose components are 1/sqrt(d) (0.125 at d=64), against a
codebook ~50x too wide. Nearly everything falls in the innermost cells, so the
codebook collapses to a few levels and every component decodes too large:

d	bits	distinct codes used (before D102408184 -> after)
64	4	16/16 -> 4/16
768	4	16/16 -> 2/16
768	8	256/256 -> 32/256

Gets worse as d grows.

Fix

Give populate_lloyd_max_trained a scale argument that multiplies the table.
The five tqmse cases pass 1/sqrt(d) (the standard deviation of a unit-norm
vector's components) so the codebook once again (like 1.14.2) matches the data it encodes.

No change for _eden / _tq. They pass no scale, so it defaults to 1
and every table entry is bit-identical to today.

Scaling trained rather than the encode path is what keeps this a one-line fix
per callsite: every SIMD specialization and distance computer reads
this->centroids, a pointer into trained.

Notes

• NOT A REVERT TO 1.14.2, but that is fine. 1.14.2 trained on the exact
  unit-sphere marginal; this scales a Gaussian approximation of it, so centroids
  differ by up to ~20% at 8 bits. The approximation is not worse: max component
  error is lower than 1.14.2 and recall is restored (see test plan). The only
  consequence is that tqmse codes are not comparable across 1.14.2 / 1.14.3 /
  this version -- harmless, because the codebook ships with the index (below).
• SERIALIZATION STILL COMPATIBLE: Existing 1.14.3-written tqmse indexes stay readable and self-consistent:
  read_ScalarQuantizer loads trained verbatim, so decode matches how they
  were encoded, and trained length is unchanged so size validation still
  passes. They are degraded, not corrupt, and need re-indexing to benefit. No
  version guard added.

Differential Revision: D115440646

[meta-codesync]

@mnorris11 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D115440646.

[meta-codesync]

This pull request has been merged in e2f9cca.