Change FileProgress callback to channel-based progress

Replace callback-based progress reporting in Builder.AddFile with
channel-based FileHashProgress for consistency with EnumerateStatus
and ScanStatus patterns. Update scanner.go to use the new channel API.

README.md

@@ -112,6 +112,7 @@ Reading file contents and computing cryptographic hashes for manifest generation
     - `TotalBytes int64`
     - `ScannedBytes int64`
     - `BytesPerSec float64`
+    - `ETA time.Duration`
   - `FileEntry struct` - Represents an enumerated file
     - `Path string` - Relative path (used in manifest)
     - `AbsPath string` - Absolute path (used for reading file content)
@@ -154,6 +155,7 @@ Reading file contents and computing cryptographic hashes for manifest generation
     - `TotalBytes int64`
     - `CheckedBytes int64`
     - `BytesPerSec float64`
+    - `ETA time.Duration`
     - `Failures int64`
   - `Checker struct` - Verifies files against a manifest
 - **Functions**
@@ -352,15 +354,8 @@ The manifest file would do several important things:
 
 # TODO
 
-## High Priority
-
-- [ ] **Implement `fetch` command** - Currently panics with "not implemented". Should download a manifest and its referenced files from a URL.
-- [ ] **Fix import in fetch.go** - Uses `github.com/apex/log` directly instead of `internal/log`, violating codebase conventions.
-
 ## Medium Priority
 
-- [ ] **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()`) 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.