feat(ffi): add release-ffi profile and publish guard for cdylib

Workspace Cargo.toml gains [profile.release-ffi] inheriting from
release with panic = "unwind". Cargo rejects per-package `panic`
overrides (error: panic may not be specified in a package profile —
verified experimentally), so a dedicated profile is the Cargo-supported
path to build the cdylib with unwinding while the CLI stays on
panic = "abort".

crates/ffi/Cargo.toml gains:
- publish = false — prevents accidental `cargo publish` of the cdylib
  crate until the distribution plan intentionally removes the guard.
- libc = "0.2" under the macos target — Unit 11 uses pthread_main_np().

Both profiles coexist:
- target/release/libagent_desktop_ffi.dylib       442 KB (panic=abort)
- target/release-ffi/libagent_desktop_ffi.dylib   464 KB (panic=unwind)

Size delta for unwind metadata (~22 KB, under the 500 KB risk budget).
Prerequisite for Unit 1's panic-boundary macro.

Cargo.toml

@@ -25,3 +25,7 @@ lto           = true
 codegen-units = 1
 strip         = true
 panic         = "abort"
+
+[profile.release-ffi]
+inherits = "release"
+panic    = "unwind"

crates/ffi/Cargo.toml

@@ -3,6 +3,7 @@ name    = "agent-desktop-ffi"
 version.workspace = true
 edition.workspace = true
 license.workspace = true
+publish = false
 
 [lib]
 crate-type = ["cdylib"]
@@ -12,6 +13,7 @@ agent-desktop-core.workspace = true
 
 [target.'cfg(target_os = "macos")'.dependencies]
 agent-desktop-macos = { path = "../macos" }
+libc                = "0.2"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 agent-desktop-windows = { path = "../windows" }