Fix MAILBOX_SIZE constant: 256 KB → 16 KB

[Copilot]

MAILBOX_SIZE in api/src/mailbox.rs was still set to 256 KB after the mailbox was reduced to 16 KB, causing MAX_CM_SHA_INPUT_SIZE (derived as MAILBOX_SIZE - 12) to be ~262 KB instead of the correct ~16 KB.

Changes

• api/src/mailbox.rs
  • MAILBOX_SIZE: 256 * 1024 → 16 * 1024, comment updated to match
  • MAX_CM_SHA_INPUT_SIZE: implicitly fixed to 16372 (was 262132) since it derives from MAILBOX_SIZE
  • mbox_write_fifo local constant: renamed MAILBOX_SIZE → PASSIVE_MAILBOX_SIZE (value stays 256 * 1024) — this function is called in passive mode for large writes like firmware load and must retain the hardware FIFO capacity, distinct from the software API mailbox size

// Before
/// Mailbox size in bytes (256 KB).
pub const MAILBOX_SIZE: usize = 256 * 1024;

// After
/// Mailbox size in bytes (16 KB).
pub const MAILBOX_SIZE: usize = 16 * 1024;

[linux-foundation-easycla]

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