feat(lang): block value requires no trailing ; (Rust-style)

A block's value is now its last statement ONLY when that statement is a
trailing expression with no `;`. A trailing `;` discards the value,
leaving the block void. This makes value-vs-statement explicit and lets
the compiler reject "this block was supposed to produce a value".

Compiler:
- Parser records `Block.produces_value` (last stmt is a no-`;` trailing
  expression) + `Block.discarded_semi` (the `;` that discarded a value),
  via `expectSemicolonAfter`. A trailing expression before `}` may now
  omit its `;` (previously a parse error). Match-arm and else-arm bodies
  are built value-producing regardless of the arm `;` (arms are exempt —
  the `;` is an arm terminator).
- Lowering: `lowerBlockValue` / the block-expr path / `inferExprType`
  respect `produces_value`. A value-position block that discards its value
  is a hard error (`lowerValueBody` for function bodies; the value-context
  `.block` path for if/else branches, `catch` bodies, value bindings,
  match arms). Pure-failable `-> !` bodies (value rides the error channel)
  and a value-if whose branches are void are handled without false errors.
- `defer`/`onfail` cleanup bodies lower as statements (void), so a
  trailing `;` there is fine.

Migration (behavior-preserving — output unchanged):
- stdlib + ~210 examples: dropped the trailing `;` on value-position last
  expressions. `format` now ends with an explicit `#insert "return
  result;"` (it relied on `#insert`-as-block-value, which `;` discards).
- Two `main :: () -> s32` examples that relied on the old silent
  default-return got an explicit trailing `0`.
- Rejection snapshots 0412 / 1013 regenerated (their quoted source lines
  lost a `;`); the diagnostics themselves are unchanged.

Docs/tests: specs.md "Block values" section; examples 0040 (rules) + 0041
(rejection); 3 parser unit tests. Filed issue 0066 (pre-existing
match-arm negated-literal phi-width quirk, surfaced not caused here).

Gates: zig build, zig build test, run_examples.sh -> 343 passed,
cross_compile.sh -> 7 passed (also refreshed its stale example names).

library/modules/allocators.sx

@@ -42,7 +42,7 @@ GPA :: struct {
     alloc_count: s64;
 
     init :: () -> GPA {
-        GPA.{ alloc_count = 0 };
+        GPA.{ alloc_count = 0 }
     }
 }
 
@@ -96,7 +96,7 @@ Arena :: struct {
     init :: (parent_alloc: Allocator, size: s64) -> Arena {
         self : Arena = .{ first = null, end_index = 0, parent = parent_alloc };
         self.add_chunk(size);
-        self;
+        self
     }
 
     reset :: (a: *Arena) {
@@ -140,7 +140,7 @@ impl Allocator for Arena {
         buf : [*]u8 = xx self.first;
         ptr := @buf[16 + self.end_index];
         self.end_index = self.end_index + aligned;
-        ptr;
+        ptr
     }
     dealloc :: (self: *Arena, ptr: *void) {}
 }
@@ -165,7 +165,7 @@ BufAlloc :: struct {
         b.buf = @buf[self_size];
         b.len = len - self_size;
         b.pos = 0;
-        b;
+        b
     }
 
     reset :: (b: *BufAlloc) {
@@ -181,7 +181,7 @@ impl Allocator for BufAlloc {
         }
         ptr := @self.buf[self.pos];
         self.pos = self.pos + aligned;
-        ptr;
+        ptr
     }
     dealloc :: (self: *BufAlloc, ptr: *void) {}
 }
@@ -221,11 +221,11 @@ TrackingAllocator :: struct {
             alloc_count = 0,
             dealloc_count = 0,
             total_alloc_bytes = 0,
-        };
+        }
     }
 
     leak_count :: (t: *TrackingAllocator) -> s64 {
-        t.alloc_count - t.dealloc_count;
+        t.alloc_count - t.dealloc_count
     }
 
     report :: (t: *TrackingAllocator) {
@@ -241,7 +241,7 @@ impl Allocator for TrackingAllocator {
             self.alloc_count += 1;
             self.total_alloc_bytes += size;
         }
-        ptr;
+        ptr
     }
     dealloc :: (self: *TrackingAllocator, ptr: *void) {
         self.parent.dealloc(ptr);