Extract shared index-validation and j-handling helpers used by `[.tf` and `[.tf_mv`

[fabian-s]

Follow-up to #252. The narrow fix for #252 added an inline vec_as_location(..., missing = "error") and an inform-dedup to [.tf_mv/tf_evaluate.tf_mv, but the matrix-i decomposition, the j-normalization, and the missing(j) default branch are duplicated across R/brackets.R ([.tf) and R/brackets-mv.R ([.tf_mv).

The reviewer of PR for #252 confirmed: "[.tf interleaves matrix-i decomp, vec_as_location, the missing(j) default branch, j-normalisation, and NA-fill on shared locals; a clean shared helper is genuinely invasive."

This issue tracks the extraction. Suggested shape:

# R/brackets.R
tf_validate_i <- function(i, x_size) {
  vec_as_location(i, n = x_size, missing = "error")
}
tf_normalize_j <- function(j, arg, ...) {
  # handle matrix-i, missing-j, value vs position semantics
}

Used by both [.tf and [.tf_mv. After extraction, the inline workaround in [.tf_mv can be removed and replaced with a call.

Important non-regression criterion (user explicitly said so during review): the four-mode behavior of [.tf (x[i] → tf, x[i, j] → matrix, x[i, j, matrix = FALSE] → list of data frames, x[cbind(i, j)] → numeric) is pet-design and intentional, baked in via backwards compatibility, and must be preserved. The shared helpers must not collapse or simplify this dispatch — they are extraction-only.

Found while reviewing PR for #252.