generate_corr() produces invalid correlation matrix for k > 2 analyses

@keaven is already working on this.

Since Apr 7, 2026.

## Bug description

`generate_corr()` produces an invalid correlation matrix (with entries > 1 and incorrect 0s) when the number of analyses `k > 2`. The function works correctly for `k = 2`.

## Root cause

The within-hypothesis across-analysis loop only fills entries relative to analysis 1, and uses the wrong event count for non-adjacent analysis pairs:

```r
# Current code (line ~14-20 of generate_corr):
for (i in 1:n_hypotheses) {
for (j in 2:n_analyses) {
count <- as.numeric(event %>% filter(H1 == i & H2 == i & Analysis == j - 1) %>% select(Event))
D[i, n_hypotheses * (j - 1) + i] <- count
D[n_hypotheses * (j - 1) + i, i] <- count
}
}
```

**Problem 1**: This loop only fills `D[row_at_A1, col_at_Aj]` — it never fills entries between non-first analyses (e.g., `D[H1_A2, H1_A3]` is left as 0).

**Problem 2**: For `j = 3` (analysis 3), `Analysis == j - 1` fetches events at analysis 2, but the correct value for `D[H1_A1, H1_A3]` should be events at `min(1, 3) = analysis 1`.

## Reproducible example

```r
library(wpgsd)
library(tibble)

# 2 hypotheses, 3 analyses
event <- tribble(
~H1, ~H2, ~Analysis, ~Event,
1, 1, 1, 50, 2, 2, 1, 80, 1, 2, 1, 50,
1, 1, 2, 80, 2, 2, 2, 130, 1, 2, 2, 80,
1, 1, 3, 100, 2, 2, 3, 170, 1, 2, 3, 100
)

corr <- generate_corr(event)
round(corr, 3)
#> H1_A1 H2_A1 H1_A2 H2_A2 H1_A3 H2_A3
#> [1,] 1.000 0.791 0.791 0.620 1.131 0.542 # <-- 1.131 > 1 (invalid!)
#> [2,] 0.791 1.000 0.625 0.784 0.559 1.115 # <-- 1.115 > 1 (invalid!)
#> [3,] 0.791 0.625 1.000 0.784 0.000 0.686 # <-- 0.000 (should be ~0.894)
#> [4,] 0.620 0.784 0.784 1.000 0.702 0.000 # <-- 0.000 (should be ~0.875)
#> [5,] 1.131 0.559 0.000 0.702 1.000 0.767
#> [6,] 0.542 1.115 0.686 0.000 0.767 1.000

# Entries > 1 make this invalid as a correlation matrix
# Entries = 0 between H1_A2<->H1_A3 are wrong (should be sqrt(80/100) = 0.894)
```

## Proposed fix

Replace the within-hypothesis loop with one that fills all analysis pairs using events at `min(s, t)`:

```r
for (i in 1:n_hypotheses) {
for (s in 1:n_analyses) {
for (t in 1:n_analyses) {
if (s == t) next
count <- as.numeric(event %>%
filter(H1 == i & H2 == i & Analysis == min(s, t)) %>%
select(Event))
row_idx <- n_hypotheses * (s - 1) + i
col_idx <- n_hypotheses * (t - 1) + i
D[row_idx, col_idx] <- count
}
}
}
```

With this fix, the same example produces a valid positive-definite correlation matrix:

```
H1_A1 H2_A1 H1_A2 H2_A2 H1_A3 H2_A3
[1,] 1.0000 0.7906 0.7906 0.6202 0.7071 0.5423
[2,] 0.7906 1.0000 0.6250 0.7845 0.5590 0.6860
[3,] 0.7906 0.6250 1.0000 0.7845 0.8944 0.6860
[4,] 0.6202 0.7845 0.7845 1.0000 0.7016 0.8745
[5,] 0.7071 0.5590 0.8944 0.7016 1.0000 0.7670
[6,] 0.5423 0.6860 0.6860 0.8745 0.7670 1.0000
```

## Impact

This bug affects any use of `generate_corr()` followed by `generate_bounds()` for designs with 3 or more analyses. The existing tests only cover `k = 2`, so the bug was not caught.