Fix: Buffer.from() in CRC helpers prevents browser/Deno/Bun execution (#444)

The Web APIs migration in c78b720 removed all Node.js Buffer usage so the
decoder runs cleanly in browser, Deno, Bun, and edge runtimes — but two
calls in the CRC-16 helpers (crc16IbmSdlcRev and crc16Genibus) were
missed. Both still called `Buffer.from(data, 'ascii')`, which is
undefined outside Node and would throw `ReferenceError: Buffer is not
defined` when ARINC 702 H1 checksum validation runs in non-Node
environments.

Fix: replace with a small inline `asciiStringToBytes()` helper that
walks the string and writes one byte per char into a Uint8Array (exactly
what `Buffer.from(s, 'ascii')` does). Same per-char masking with 0xff so
the byte values are identical.

Surfaced while centralizing the lib/utils/ helpers into the
airframes-decoder repo's runtimes/typescript/ for the upcoming
cross-language unification work; the runtime package's stricter tsconfig
caught the surviving Buffer references that the original repo's looser
config did not.

Verified: 23 Label_H1 test suites green (115/117 tests pass, 2 skipped
matching baseline).

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Mark Bumiller <mark.bumiller@gmail.com>

Author: 3 people

lib/utils/arinc_702_helper.ts

@@ -7,6 +7,15 @@ import { FlightPlanUtils } from './flight_plan_utils';
 import { ResultFormatter } from './result_formatter';
 import { RouteUtils } from './route_utils';
 
+// One byte per ASCII char — replaces the Buffer.from(data, 'ascii') idiom so
+// the CRC helpers run in browser / Deno / Bun / edge runtimes without Node
+// polyfills. Matches the broader Web APIs migration in c78b720.
+function asciiStringToBytes(data: string): Uint8Array {
+  const bytes = new Uint8Array(data.length);
+  for (let i = 0; i < data.length; i++) bytes[i] = data.charCodeAt(i) & 0xff;
+  return bytes;
+}
+
 /**
  * Helper class for decoding ARINC 702 messages
  * contains core logic for decoding different types of ARINC 702 messages,
@@ -658,7 +667,7 @@ function processSummary(decodeResult: DecodeResult, data: string[]) {
 // CRC-16/IBM-SDLC but nibbles are reversed
 function crc16IbmSdlcRev(data: string): number {
   let crc = 0xffff;
-  const bytes = Buffer.from(data, 'ascii');
+  const bytes = asciiStringToBytes(data);
 
   for (const byte of bytes) {
     crc ^= byte;
@@ -689,7 +698,7 @@ function crc16Genibus(data: string): number {
   let crc = 0xffff;
   const polynomial = 0x1021;
 
-  const bytes = Buffer.from(data, 'ascii');
+  const bytes = asciiStringToBytes(data);
 
   for (const byte of bytes) {
     crc ^= byte << 8;