Refuse a bad COMPUTER_MEMORY_BYTES instead of capping a computer at 512 bytes

[Bartok9]

What this changes

COMPUTER_MEMORY_BYTES=512m used to become 512 via parseInt, which Docker accepts as a memory cap Chromium cannot live in. Empty COMPUTER_MEMORY_BYTES= is still unset (no cap). Anything that is not a whole number of bytes now exits the supervisor before a computer is created — same empty-string / prefix trap as PORT (#343).

Where it runs

• New state that outlives a request? None. Process env at supervisor start.
• What happens on the second replica? Each supervisor process parses its own env the same way. No shared state.
• Anything serialised? No.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? No.

Boundary and audit

• Gateway path unchanged (supervisor boot only).
• No new refusals on acting calls.
• Nothing new from the client.

Changelog

• Unreleased line: a bad COMPUTER_MEMORY_BYTES refuses to start.

Proof

bun is not on this machine. Tests added in supervisor/tests/computer-memory-bytes.test.ts matching listen-port.test.ts.