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/process.sx

@@ -78,7 +78,7 @@ run :: (cmd: [:0]u8) -> ?ProcessResult {
             exit_code = 128 + (raw_status & 0x7F);
         }
     }
-    ProcessResult.{ exit_code = exit_code, stdout = out };
+    ProcessResult.{ exit_code = exit_code, stdout = out }
 }
 
 // Read an environment variable. Returns null if unset; an empty
@@ -91,7 +91,7 @@ env :: (name: [:0]u8) -> ?string {
     if n == 0 { return ""; }
     buf := cstring(cast(s64) n);
     memcpy(buf.ptr, xx p, cast(s64) n);
-    buf;
+    buf
 }
 
 // Locate an executable by walking `$PATH`. Returns the absolute path
@@ -115,7 +115,7 @@ find_executable :: (name: [:0]u8) -> ?string {
         if out.len == 0 { return null; }
         return out;
     }
-    null;
+    null
 }
 
 // ── Process termination (ERR step E4.1) ───────────────────────────────
@@ -139,7 +139,7 @@ exit :: (code: u8, loc: Source_Location = #caller_location) -> noreturn {
         print("\nprocess.exit({}) called from {} at {}:{}\n", code, loc.func, loc.file, loc.line);
         trace.print_interpreter_frames();
     }
-    clib_exit(xx code);
+    clib_exit(xx code)
 }
 
 // Abort with a message when `cond` is false. Prints `ASSERTION FAILED at