Remove generateInner, now handled by Builder

- Remove generateInner() from serialize.go
- Update generate() to error if pbInner not set
- Remove legacy tests that depended on old code path
- Update TODO item to reflect removal

README.md

@@ -362,7 +362,7 @@ The manifest file would do several important things:
 - [ ] **Add `--force` flag for overwrites** - Currently silently overwrites existing manifest files. Should require `-f` to overwrite.
 - [ ] **Implement FollowSymLinks option** - The flag exists in CLI and Options structs but does nothing. Scanner should use `EvalSymlinks` or `Lstat`.
 - [ ] **Change FileProgress callback to channel** - `mfer/builder.go` uses a callback for progress reporting; should use channels like `EnumerateStatus` and `ScanStatus` for consistency.
-- [ ] **Consolidate legacy manifest code** - `mfer/manifest.go` has old scanning code (`Scan()`, `addFile()`, `generateInner()`) that duplicates the new `internal/scanner` + `mfer/builder.go` pattern.
+- [ ] **Consolidate legacy manifest code** - `mfer/manifest.go` has old scanning code (`Scan()`, `addFile()`) that duplicates the new `internal/scanner` + `mfer/builder.go` pattern.
 - [ ] **Add context cancellation to legacy code** - The old `manifest.Scan()` doesn't support context cancellation; the new scanner does.
 
 ## Lower Priority