lang: slice ranges take the same bound markers as for-header ranges

xs[1..=3] (end inclusive), xs[0<..<4] (both exclusive), xs[..=2]
(prefix form with markers, implicit 0 start), xs[2<..] (open end,
exclusive start), and xs[..] (whole collection) — lowered as lo+1 /
hi+1 on the existing subslice op. Strings slice through the same path.
An explicit end marker requires an end expression, matching the
for-header rule.

Regression: examples/0052-basic-slice-range-bounds.sx.

specs.md

@@ -965,9 +965,22 @@ word := msg[6..11];  // string → "world"
 - `expr[start..end]` — elements from `start` (inclusive) to `end` (exclusive)
 - `expr[start..]` — elements from `start` to end
 - `expr[..end]` — elements from beginning to `end`
+- `expr[..]` — the whole collection as a slice
 - Result type: `[]T` for arrays/slices, `string` for strings
 - No memory allocation — the result points into the original backing storage
 
+Slice ranges take the same bound markers as for-header ranges — `=`
+inclusive / `<` exclusive on either side of `..`, defaulting to
+start-inclusive, end-exclusive:
+```sx
+arr[1..=3]    // elements 1, 2, 3
+arr[0<..<4]   // elements 1, 2, 3
+arr[..=2]     // elements 0, 1, 2 (prefix form takes markers too)
+arr[2<..]     // elements 3 .. len-1
+```
+An explicit end marker (`..=` / `..<`) requires an end expression. Bounds
+are arbitrary expressions (`arr[x-1..=x+1]`).
+
 ### Pointer Types
 
 | Syntax | Meaning | `.len` | `[i]` |