lang: xx <pack> materializes a comptime pack into a runtime slice (issue 0053)

xx args with a slice target now bridges a comptime pack to a runtime slice:
[]Any boxes each element to Any; []P xx-erases each to the protocol (reusing
the slice-of-protocol erasure from 0052). New lowerPackToSlice; the unary-op
arm intercepts xx <pack> before the pack-as-value diagnostic. This is the
working forward to a runtime []Any/[]P helper -- log_count(xx args) -> 3 --
so the 2.7 pack-as-value diagnostics now suggest xx <name> for the call case.

examples/204-pack-xx-to-slice.sx (both []Any and []P paths); 203 help text
updated. issue 0053 FIXED. 239 examples + unit green.

tests/expected/203-pack-as-value.txt

@@ -15,7 +15,7 @@ error: pack 'xs' has no runtime value — a pack is comptime-only and can't be u
 15 | call    :: (..xs: Show) -> void { sink(xs); }                // B: pass to a call
    |                                        ^^
 
-help: materialize a tuple `(..xs)` to store it, or declare `xs` as a slice variadic `..xs: []P` for runtime use instead of a pack `..xs: P`
+help: materialize a tuple `(..xs)` to store it, or `xx xs` to convert it to an expected `[]Any`/`[]P` slice
    |
 15 | call    :: (..xs: Show) -> void { sink(xs); }                // B: pass to a call
    |                                        ^^

tests/expected/204-pack-xx-to-slice.exit

@@ -0,0 +1 @@
+0

tests/expected/204-pack-xx-to-slice.txt

@@ -0,0 +1,5 @@
+any=3
+A
+B
+A
+show=3