iter 24.2: class Show + 4 primitive instances + 22b TShow migration

Ships class Show a where show : (a borrow) -> Str in the prelude
plus primitive Show Int / Bool / Str / Float instances. Each
instance body is a single-application lambda invoking the
corresponding runtime primitive (int_to_str, bool_to_str,
str_clone, float_to_str) — first prelude instance bodies to call
runtime primitives directly. Float is included in Show (unlike
Eq/Ord) because textual representation is well-defined modulo the
NaN-spelling caveat at DESIGN.md §Float semantics.

The 22b typeclass test corpus is migrated preemptively: 21 fixture
files and 6 consumer files (typeclass_22b{2,3}.rs +
hash.rs ct4 pin + workspace.rs iter22b1 tests + the
instance_present.prose.txt snapshot) rename class Show / show
to class TShow / tshow, analogous to the existing TEq/TOrd
convention. Migration runs before the prelude additions so the
workspace stays green throughout.

Three new tests pin the post-mono shape: show_mono_synthesis.rs
(existence of show__Int/Bool/Str/Float as Def::Fn entries in the
prelude post-mono module), show_dispatch_pin.rs (bare show and
tshow both Step-2 singletons workspace-globally),
mono_hash_stability.rs::primitive_show_mono_symbol_hashes_stay_bit_identical
(body hashes pinned for the 4 new mono symbols).

DESIGN.md §Prelude (built-in) classes drops Show from the
deferred-features list and appends a milestone-24 paragraph
naming the class signature + the 4 instances + the body shape.
print rewire stays deferred to iter 24.3.

Tests: 552 passed (was 548 + 4 new). bench/compile_check +
bench/cross_lang exit 0. bench/check exits 1 on the recurring
noise envelope per the conservative-call lineage.

docs/DESIGN.md

@@ -1888,12 +1888,26 @@ end-to-end path is the milestone's typeclass acceptance gate
 Milestone 23 amends the above: the prelude now ships the `Ordering`
 ADT, the `Eq` and `Ord` classes, primitive `Eq Int/Bool/Str` and
 `Ord Int/Bool/Str` instances, and the five polymorphic free-fn helpers
-`ne` / `lt` / `le` / `gt` / `ge`. `Show`, operator routing through
-`Eq`/`Ord`, and `print`-rewire remain out of scope per their original
-substantive reasons (heap-`Str` ABI, bench rebaseline). Float has
-neither `Eq` nor `Ord` instance per §"Float semantics"; a polymorphic
-helper invoked at Float fires `NoInstance` at typecheck with a
-Float-aware diagnostic cross-referencing this section.
+`ne` / `lt` / `le` / `gt` / `ge`. Operator routing through `Eq`/`Ord`
+and `print`-rewire remain out of scope per their original substantive
+reasons (bench rebaseline, milestone-24 dependency on the post-mq
+dispatcher); `Show` itself ships in milestone 24. Float has neither
+`Eq` nor `Ord` instance per §"Float semantics"; a polymorphic helper
+invoked at Float fires `NoInstance` at typecheck with a Float-aware
+diagnostic cross-referencing this section.
+
+Milestone 24 amends the above further: the prelude ships `class Show
+a where show : (a borrow) -> Str` and primitive `Show Int`,
+`Show Bool`, `Show Str`, `Show Float` instances. Float **is** included
+in Show (unlike Eq/Ord) — IEEE-754 makes structural equality and total
+ordering semantically dubious, but textual representation of a Float
+is well-defined modulo the NaN-spelling caveat in §"Float semantics".
+Each `Show <T>` instance body is a single-application lambda invoking
+the corresponding runtime primitive (`int_to_str`, `bool_to_str`,
+`str_clone`, `float_to_str`); no codegen intercept is required. The
+polymorphic helper `print : forall a. Show a => a -> () !IO` ships in
+the same milestone as the second iteration (24.3) and routes through
+`show` and `io/print_str`.
 
 Primitive output goes through `io/print_int` / `io/print_bool` /
 `io/print_str` directly.