t27b: []T slices #6248

Refs #6063

type []T was the top first-rejection blocker in t27b corpus specs (136 files, 1547 occurrences). This issue tracks teaching the t27b backend the slice family: []const u8, []const T and []T params, slicing a fixed array or a slice a[i..j] / a[i..], .len, indexing with a bounds trap, and for (s) |x|.

Design: reuse the [N]T design (PR #6247). A slice is a (ptr, len) pair. Indexing is checked and traps index out of bounds. There is no interpreter fallback and no skipped tests, and anything unsupported is a clean reject that names the construct.

Stacked on claude/t27b-fixed-arrays (#6247).