P5.6 prereq: port bitwise/shift ops into the comptime VM

`comptime_vm` exec now handles `bit_and`/`bit_or`/`bit_xor`/`bit_not`/`shl`/`shr`
(a new `bitwise` helper next to `arith`), mirroring the legacy interp's i64 model
exactly: the shift amount clamps to `@min(rhs, 63)` and `shr` is an arithmetic
right shift (sign-extending).

These were unported and bailed; the `shr` gap surfaced via the iOS-device bundler
once P5.5 let it run further (1616). With the port, 1616's strict VM run reaches
the real bundler logic and stops only at the genuinely-unavailable iOS runtime on
macOS (`_UIApplicationMain` / no linked binary under `sx run`), as expected.

New corpus test `examples/0639-comptime-bitwise-shift.sx` folds AND/OR/XOR/NOT/
shl/shr/arith-shr as `::` consts — identical on both evaluators. 704/0 both gates.

examples/0639-comptime-bitwise-shift.sx

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+// Comptime bitwise + shift ops on the VM: AND/OR/XOR/NOT and shl/shr all fold
+// at compile time. Regression for the VM op port (P5.6) — these were unported in
+// `comptime_vm` and bailed (`shr` surfaced via the iOS-device bundler). `shr` is
+// an arithmetic right shift (sign-extending), mirroring the legacy interp's i64
+// model; the shift amount clamps to 63.
+
+#import "modules/std.sx";
+
+AND :: 0b1100 & 0b1010; // 0b1000 = 8
+OR  :: 0b1100 | 0b1010; // 0b1110 = 14
+XOR :: 0b1100 ^ 0b1010; // 0b0110 = 6
+NOT :: ~0;              // -1
+SHL :: 1 << 10;         // 1024
+SHR :: 1024 >> 3;       // 128
+ASHR :: (-8) >> 1;      // -4 (arithmetic, sign-extending)
+
+main :: () {
+    print("and={} or={} xor={} not={}\n", AND, OR, XOR, NOT);
+    print("shl={} shr={} ashr={}\n", SHL, SHR, ASHR);
+}

examples/expected/0639-comptime-bitwise-shift.exit

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+0

examples/expected/0639-comptime-bitwise-shift.stderr

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+

examples/expected/0639-comptime-bitwise-shift.stdout

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+and=8 or=14 xor=6 not=-1
+shl=1024 shr=128 ashr=-4