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).

examples/expected/0040-basic-block-value.exit

@@ -0,0 +1 @@
+35

examples/expected/0040-basic-block-value.stderr

@@ -0,0 +1 @@
+

examples/expected/0040-basic-block-value.stdout

@@ -0,0 +1 @@
+block-value total: 35

examples/expected/0041-basic-block-value-reject.exit

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

examples/expected/0041-basic-block-value-reject.stderr

@@ -0,0 +1,5 @@
+error: function returns 's32' but the last expression's value is discarded by this `;` — drop the `;` to return it (or use an explicit `return`)
+  --> /Users/agra/projects/sx/examples/0041-basic-block-value-reject.sx:10:10
+   |
+10 |     n * 2;
+   |          ^

examples/expected/0041-basic-block-value-reject.stdout

@@ -0,0 +1 @@
+

examples/expected/0412-protocols-impl-duplicate-same-file.stderr

@@ -3,7 +3,7 @@ error: duplicate impl 'Into' for source 's64' in /Users/agra/projects/sx/example
    |
 15 | impl Into(MyA) for s64 {
    | ^^^^^^^^^^^^^^^^^^^^^^^^
-16 |   convert :: (self: s64) -> MyA { .{ v = self * 2 }; }
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+16 |   convert :: (self: s64) -> MyA { .{ v = self * 2 } }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 17 | }
    | ^

examples/expected/1013-errors-value-failable-reject.stderr

@@ -1,5 +1,5 @@
 error: `catch` body must produce a value of type 's32' (or diverge with `return` / `raise`)
   --> /Users/agra/projects/sx/examples/1013-errors-value-failable-reject.sx:17:10
    |
-17 |     x := parse(-1) catch e { print("oops\n"); };   // error: body yields no value
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+17 |     x := parse(-1) catch e { print("oops\n") };   // error: body yields no value
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^