Add Brainfuck frontend (bfcas) to CIR

[Copilot]

Adds a Brainfuck-to-CIR compiler and bfcas runner tool, enabling Brainfuck programs to execute on the CIR VM.

VM bug fixes (prerequisite)

Two bugs in core/cir.h prevented arithmetic from working at all:

• Register flag lost on write: WordFlag::Register was cleared whenever a value was assigned to a register (via operator=), causing subsequent expect_flag(Register) guards on Add, Sub, Load, etc. to always throw. Fixed by having move() re-set the flag after assignment, and routing all register-writing ops through move().
• Inc/Dec no-op: operator++/operator-- return a new Word by value; the results were silently discarded. Fixed by assigning the return value back via move().

New stdlib functions (core/std.h)

• std.putchar — writes r0 as a character
• std.getchar — reads one byte into r0

Brainfuck compiler (core/bf.h)

BrainfuckCompiler directly emits Program objects (no assembly text round-trip). Jump targets for [/] are resolved via a two-pass stack approach: [ emits a je with a placeholder, patched when the matching ] is encountered.

Register layout:

Register	Role
r0	I/O (putchar/getchar)
r1	Data pointer (current tape cell address)
r2	Temporary cell value
r3	Tape base (freed on exit)

CLI tool (tools/bf/main.cpp)

New bfcas binary — compile and run .bf files directly:

bfcas examples/bf/hello.bf
# Hello World!

Example

examples/bf/hello.bf — Hello World in Brainfuck.

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