Add proof-backed parser byte reads and preserve retryable source failures

src/lib/canvas-runtime/import-byte-reader.ts

@@ -0,0 +1,17 @@
+import {fault} from '../canvas-core/values.ts';
+
+const failures=new Set(['import_source_changed','invalid_import_byte_range','import_read_capacity','import_source_closed','import_source_read_failed','invalid_import_read_proof','import_read_proof_interpreter_unavailable']);
+export const isImportByteFailure=(error:unknown)=>failures.has((error as {code?:string}|undefined)?.code??'');
+
+/** Optional immutable-source port. Implementations return exactly the requested bytes. */
+export type ImportByteReader={readonly byteLength:number;read(start:number,end:number,signal:AbortSignal):Promise<Uint8Array>};
+export async function readImportRange(bytes:Blob,reader:ImportByteReader|undefined,start:number,end:number,signal:AbortSignal):Promise<Uint8Array<ArrayBuffer>>{
+ if(signal.aborted)throw fault('import_aborted');
+ if(!Number.isSafeInteger(start)||!Number.isSafeInteger(end)||start<0||end<start||start>bytes.size||reader&&reader.byteLength!==bytes.size)throw fault('invalid_import_byte_range');
+ const limit=Math.min(end,bytes.size);let value:Uint8Array;
+ try{value=reader?await reader.read(start,limit,signal):new Uint8Array(await bytes.slice(start,end).arrayBuffer());}
+ catch(error){if(signal.aborted)throw fault('import_aborted');if(isImportByteFailure(error))throw error;throw fault('import_source_read_failed');}
+ if(signal.aborted)throw fault('import_aborted');
+ if(!(value instanceof Uint8Array)||value.byteLength!==limit-start)throw fault('import_source_changed');
+ return reader?new Uint8Array(value):value as Uint8Array<ArrayBuffer>;
+}