AcadianGY 12.3.9: ops\$CSI_SCALE knob for climate weighting

[holoros]

Adds one optional knob to AcadianGYOneStand: `ops$CSI_SCALE`. When set, multiplies the resolved Climate Site Index by the provided factor before anything downstream uses it. Default unset; 12.3.9 with no CSI_SCALE in ops is byte-identical to 12.3.8.

The change

In AcadianGYOneStand, immediately after the CSI resolution line:

```r
CSI = if (is.null(stand) || is.null(stand$CSI) || is.na(stand$CSI)) 12 else stand$CSI
if (!is.null(ops$CSI_SCALE) && is.finite(ops$CSI_SCALE) && ops$CSI_SCALE > 0) {
CSI <- CSI * ops$CSI_SCALE
}
```

Version tag: AcadianV12.3.9.

v28 validation (100 ME FIA plots, 10 yr, MORTCAL + ingrowth on)

CSI_SCALE	BA bias	R^2	TPA (obs 1029)	QMD (obs 4.97)
1.0 (unset, = v24)	+11.05%	0.4232	1043	4.92
0.7 (production recommendation)	+9.96%	0.4325	1035	4.94
0.5 (aggressive)	+9.04%	0.4411	1029	4.95

CSI_SCALE unset reproduces v24 exactly. CSI_SCALE=0.7 matches the v25 sensitivity interpolation. CSI_SCALE=0.5 lands TPA at observed and pulls BA bias below +10% for the first time.

Recommendations

• Customrun bridge default: CSI_SCALE=0.7 — ~1 pp BA closure, +0.01 R^2, low risk
• Optional aggressive: CSI_SCALE=0.5 — ~2 pp closure, watch for over-tuning
• Canadian MAGPlot: leave unset — no residual climate signal to close on NB CFI

Deployed to fvsOL/inst/extdata/AcadianGY.R with 12.3.8 backed up alongside the earlier 12.3.5/12.3.6/12.3.7 backups.