refactor: replace postcard/serde with fixed-size LE wire format

Drop postcard and serde dependencies entirely. All integers are now
fixed-width little-endian on the wire — no varints, no zigzag encoding.
A developer sending tx_power_dbm=14 sees 0x0E on the wire. No surprises.

- RadioConfig: 10 bytes fixed (freq:4 LE, bw:1, sf:1, cr:1, sync:2 LE, pwr:1)
- Commands/Responses: tag byte + fixed-size fields
- Vec<u8>: 2-byte LE length prefix + raw bytes
- Option<T>: 1-byte (0=None, 1=Some) + T
- COBS framing preserved (just byte-stuffing, no encoding tricks)
- Manual COBS decode replaces postcard's CobsAccumulator
- Removes ~15KB from firmware binary (no serde monomorphization)

Cargo.lock

@@ -336,6 +336,7 @@ name = "donglora"
 version = "0.1.0"
 dependencies = [
  "cfg-if",
+ "cobs",
  "cortex-m-rt",
  "critical-section",
  "defmt 0.3.100",
@@ -359,8 +360,6 @@ dependencies = [
  "minijinja",
  "owo-colors",
  "panic-probe",
- "postcard",
- "serde",
  "ssd1306",
  "static_cell",
 ]
@@ -1026,7 +1025,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0bfb9eb618601c89945a70e254898da93b13be0388091d42117462b265bb3fad"
 dependencies = [
  "hash32",
- "serde",
  "stable_deref_trait",
 ]
 
@@ -1353,16 +1351,6 @@ version = "1.13.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c33a9471896f1c69cecef8d20cbe2f7accd12527ce60845ff44c153bb2a21b49"
 
-[[package]]
-name = "postcard"
-version = "1.1.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6764c3b5dd454e283a30e6dfe78e9b31096d9e32036b5d1eaac7a6119ccb9a24"
-dependencies = [
- "cobs",
- "serde",
-]
-
 [[package]]
 name = "proc-macro-crate"
 version = "3.2.0"